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Zoology Publications from Victoria University of Wellington—Nos. 76 and 77

Nasolamia, New Genus — Type Species. Carcharhinus velox Gilbert, 1898

Nasolamia, New Genus
Type Species. Carcharhinus velox Gilbert, 1898.

Nasolamia is unique in the Carcharhinidae in having very large, transverse, close-set nostrils with very large, oval excurrent apertures and with the internarial space 1.1 to 1.3 times the nostril width. Other carcharhinid genera have smaller nostrils, with oblique apertures, narrow, slitlike excurrent apertures, and internarial spaces 3 to 4 or more times the nostril width. The narrow snout and peculiar nostrils of Nasolamia velox are very striking and conspicuous, as one of us (Compagno) noted while sorting hundreds of carcharhinid heads at a fishmeal plant at Guaymas, Sonora, Mexico.

Cranially Nasolamia differs from other carcharhinid genera by differences in the ethmoid region. The narrow external internarial space of Nasolamia is reflected by the narrow space between the nasal apertures of the cranium (Figs. 2-3), which is less than the nasal aperture width. In other carcharhinid genera (except Prionace), the space between the nasal apertures is equal to or greater than the nasal aperture width (Fig. 1). Nasolamia has the long axes of the nasal capsules nearly parallel to the longitudinal axis of the cranium, while in other carcharhinid genera they vary from diagonal to transverse to the cranial axis. The extreme anterioposterior elongation of the nasal capsules of Nasolamia is approached only by Prionace among other carcharhinids.

GENERIC DEFINITION: Head elongated, strikingly conical, not greatly depressed, its depth at eyes about 2/3 its width at eyes, its length about ¼ of total length. Eyes subcircular in shape, with their ventral edges at level of nostril or slightly above. No posterior eye notch. Spiracles absent. No gillrakers.

Nostrils wide, close together, nostril width 75-90% of internarial width (Fig. 4b). Long axis of nostrils transverse to body axis. Anterior nasal flap weakly developed as a low trianguar lobe, not tubular (Fig. 4c). Nostrils nearly equidistant between snout tip and mouth, but slightly closer to mouth. Excurrent aperture of nostril large, subquadrate, and revealing interior of olfactory organ in ventral view.

Labial furrows very short, virtually confined to mouth corners, uppers 0.3% to 1.0% of total length or 1/3 to 2/5 of nostril width; lower labial furrows about length of uppers and concealed by upper lip when mouth is closed. Anterior end of upper labial furrow posterior to eye about 12/3 eye lengths. Upper and lower labial cartilages present.

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Dignathic heterodonty strongly developed (Fig. 5). Upper teeth generally larger than opposite lowers, with thicker and more oblique cusps (except for the first one to two anteriors, which have erect cusps), postlateral blades only, and convex premedial edges (Fig. 5). Lowers with slenderer and more erect cusps than uppers opposite to them (however, all lower rows except the first few have slightly oblique cusps), with both premedial and postlateral blades present, and well-defined notches between cusps and blades. Lower teeth crowns proportionately lower relative to their root widths compared with upper teeth; and lower teeth roots slightly lower than those of uppers.

Tooth row groups present include distinct medials or alternates, lower symphysials, and poorly differentiated upper anteriors, the remaining parasymphysial teeth being lateroposteriors in the upper jaw and anteroposteriors in the lower jaw. Tooth rows relatively few, 27-30/24-28. Tooth formulae: 13-14 + 1-2 + 13-14/12-13 + 0-2 + 12-13; or alternates or medials 1-2, anteriors 2, lateroposteriors 11-12/medials 0-2, symphysials 1, anteroposteriors 11-12; 1 to 2 series of teeth functional in both jaws. No basal ledges, basal grooves, or transverse ridges on tooth crowns. Cusplets or coarse serrations absent from teeth, but teeth of both jaws finely serrated at all growth stages. Roots of teeth high, narrow, compressed. Strong transverse grooves and notches present on attachment surfaces of roots.

Trunk not greatly compressed. No interdorsal ridge, though preservation may give the appearance of a feeble ridge. Lateral dermal keels absent from the caudal peduncle. Both precaudal pits present, of crescentic type.

Denticles from dorsum below first dorsal fin with crowns as wide as long at all stages (Fig. 6). Denticle crowns lack cusps but have ragged posterior edges. Three to seven longitudinal ridges present on crown, with 1 medial ridge and 1-3 pairs of lateral ridges; lateral ridges fewer in young, more in large specimens. All ridges about equal in height, medial not greatly higher than laterals.

Pectoral fins moderately broad, their lengths from origins to free rear tips about 70-75% of their anterior margin lengths. Apex of adpressed pectoral slightly posterior to its free rear tip when pectoral inner margin is held parallel to body axis. Origin of pectorals under 3rd gill opening or below interspace between 3rd and 4th.

Pelvic fins relatively small, anterior margins only 32-37% of pectoral anterior margins.

Claspers with pseudosiphon and pseudopera well developed. Cover rhipidion very large, nearly reaching tip of clasper. Rhipidion present and very well-developed.

Midpoint of first dorsal base much closer to pectoral bases than to pelvic bases. Origin of first dorsal above middle of inner margin of pectoral. Fee rear tip of first dorsal slightly anterior to pelvic origins. Second dorsal much smaller than first, height 22-31% of first dorsal height. Second dorsal margin only weakly concave.

Anal fin slightly larger than second dorsal, anal height 120-170% of second dorsal height, anal base about 125% of second dorsal base. Posterior margin of anal deeply concave, anal apex and distal parts of anterior and posterior margins forming a distinct lobe. Anal origin slightly anterior to second dorsal origin or under it; anal insertion posterior to second dorsal insertion by 1/7 of anal base or less. Anal base without preanal ridges (paired extensions of the anal fin base anterior to the anal fin proper and best developed in Rhizoprionodon and Loxodon).

Postventral caudal margin deeply notched and divided into upper and lower parts. The lower postventral margin slants posterodorsally from the tip of the ventral caudal lobe to the notch and forms a right angle with the upper postventral margin. Lateral undulations on dorsal caudal margin well-developed.

Cranium with rostrum not encrusted by masses of dense, calcified cartilage. Nasal capsules subovate in shape, elongated anterioposteriorly (Fig. 2-3), very narrow relative to their length; greatest width of each nasal capsule only 2/3 of its length. Width of cranium across nasal capsules only 1¾ to 2 times in nasobasal length (distance from page 5base of medial rostral cartilage to fused vertebral half-centrum between occipital condyles). Long axis of nasal capsule nearly parallel with longitudinal cranial axis. Distance between nasal apertures 2/5 of greatest width of nasal apertures. Nasal fenestrae and lateral ectethmoid foramina absent, medial ectethmoid foramina present on posterodorsal surfaces of ectethmoid condyles. Epiphysial (pineal) foramen present. Basal plate slightly narrower than width of orbit above it at orbital notches. No suborbital ledge connecting suborbital shelf and nasal capsules. Stapedial arteries enter orbits through a pair of large fenestrae in the suborbital shelf. Internal carotid arteries enter cranium through ventral foramina in the basal plate. The internal carotid foramina are far apart from each other and close to the medial rim of each stapedial fenestra. Preorbital processes strong, bladelike; postorbital processes short. Otic capsule expanded, ovate, inflated, length about 21/3 to 2½ in nasobasal length. Postorbital processes originating at mid-length of otic capsules.

Vertebrae moderately numerous, total counts 176-183 (N = 5, mean = 179.60, standard deviation = 3.36, coefficient of variation 1.87). Monospondylic precaudal centra 28.1-28.9%, diplospondylic precaudal centra 19.2-20.2%, and diplospondylic caudal centra 51.7-52.5% of total counts (N = 4). Division between monospondylic precaudal and diplospondylic precaudal centra occurs above or just below middle of pelvic bases and is well-defined. "A" ratios (length of penultimate monospondylic centrum/length of first diplospondylic centrum x 100) are 162-190, "B" ratios (length of penultimate monospondylic centrum/its width x 100) are 90-107. Last few monospondylic precaudal centra not greatly enlarged. No 'stutter zone' of alternating long and short centra in diplospondylic precaudal or caudal region.

Vertebral centra with short diagonal calcified lamellae.

Development viviparous.

RELATIONSHIPS OF NASOLAMIA: Revisions of the family Carcharhinidae and the genus Carcharhinus by Compagno (1979, and in press) and Garrick (1982) recognise 25 or 31 species in Carcharhinus depending on whether species of the nominal genera Aprionodon and Hypoprion and the species Carcharhinus gangeticus and C. glyphis are included (Compagno) or excluded (Garrick) from Carcharhinus. The removal of Carcharhinus velox Gilbert, 1898 from Carcharhinus and its placement in the genus Nasolamia are part of these revisions.

Aside from its narial and cranial pecularities, Nasolamia has little to distinguish it from Carcharhinus and probably is a specialised derivative of one of the species groups within Carcharhinus. In dentition, vertebral counts, and general external morphology Nasolamia velox seems closest to Carcharhinus acronotus (Poey, 1860).