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A New Species of Myxine (Cyclostomata) from Cook Strait

Myxine biniplicata sp. nov

Myxine biniplicata sp. nov.

Slender, essentially subcylindrical in the vicinity of the branchial apertures, slightly compressed anteriorly so that the head is ovoidal in section and increasingly compressed posteriorly to the strongly compressed tail, which has symmetrical margins and is bluntly rounded. The head and branchial region, as measured from the tip of the rostrum to the branchial aperture, together equal 25.5% of the total length: the abdominal region, 59%; the caudal region, 16.8%. The rostrum is slightly wider at the base than long, narrowing anteriorly to the obliquely truncate extremity. The naris is guarded above by a thin membranous hood which terminates obliquely, the margin slanting anteriorly. The naris and mouth are distinct; but the lip is elevated, fleshy, much folded, ridged, and deeply grooved. A median groove simulates an oronasal fissure extending from posterior to the nasal barbels, which are flattened and rise one on either side of the base of the nasal hood, between the subnasal barbels, and widens between the bases of the lateral barbels, to narrow again before entering the oral hood between the prominent labial mounds, which each bear a short, broad-based labial "tentacle." The barbels are subequal, the posterior laterals slightly the larger; and all with black tips. The mouth is a small triangular aperture concealed by the heavy, folded lip. The skin in the anterior quarter of the head region is relatively smooth, but definitely wrinkled posterior to this and over the remainder of the body. This transition is at the level of the first mucous sacs, which are difficult to detect externally, since the outline of the individual sacs is not obvious, but dissection shows 44 are present anterior to the branchial apertures. There is no external indication of the eyes.

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The two branchial apertures are paramedian in position and at the same level. The left is slightly but obviously larger. The ventral fin commences in the branchial region 51.5 mm. from the tip of the rostrum, about half-way between the lip and the branchial apertures, as a low ridge increasing gradually in height to 1.5 mm. as it passes between the branchial apertures. Anterior to the definitive origin there are two minute diverging ridges which can be traced for 7.5 mm. and appear as anterior extensions of this fin; but they may be a result from preservation. The ventral fin continues as a low thick fold of uniform height back to the cloacal aperture, where it appears to split, forming labia to the aperture which are continuous with the caudal fin.

The lateral fins are more of the nature of a true fold, and apparently lack supporting elements. They rise each at about the same level, nearly one-third of the distance between the origin of the ventral fin and the branchial apertures, just below the line of mucous gland pores. They commence in a low ridge which can be traced anteriorly in an indefinite ridge, but posteriorly each fin increases gradually in height to about 2.5 mm., which is maintained to the level of the branchial apertures, when the fin diminishes rapidly and terminates posterior to the branchial apertures and does not meet with the ventral fin.

The abdominal region, between the branchial apertures and including the cloaca, constitutes more than half of the total length. It is slightly compressed to a sub-circular form anteriorly and increasing so posteriorly. There is no dorsal fin throughout this region, the mid-dorsal line being marked by a low ridge. The line of mucous sacs is poorly marked on the anterior region, where it is situated about three-quarters down on the side of the body; but for the posterior four-fifths of the abdomen the line is situated close to the basis of the ventral fin and the outline of the individual sacs is clearly marked above and below. The ventral fin is slightly higher anteriorly (4.5 cm.), and is of a relatively uniform height (3.5 mm.) throughout its length, diminishing rapidly in the vicinity of the cloaca. It is correctly a "fin-fold"; no skeletal elements were detected. The cloaca is slitlike and measures 5.0 mm. in length.

The tail region is the shortest and most compressed. It is fairly uniform in depth for the first half of its length, and tapers symmetrically in the latter half to terminate obtusely. The ventral lobe of the caudal fin commences at the cloaca, where it is the highest (5.5 mm.), diminishing slightly in height posteriorly, and is continuous around the end of the tail, with the dorsal lobe, which is slightly lower and extends anteriorly only about half-way to the cloaca, being more rounded, softer, and fleshy anteriorly. There are 94 elements supporting the ventral lobe of the caudal fin. The anterior two-thirds of the dorsal lobe is unsupported, the posterior third is supported by 58 elements. None of the elements is bifurcated. The line of mucous glands continues along the basis of the ventral lobe and becomes less obvious as it approaches the tip of the tail. The last sac is situated at three-quarters of the length of the tail from the cloaca.

The row of mucous gland pores commences on the head at about the usual level, 26% of the distance from the tip of the rostrum to the branchial apertures. The glands are arranged as 44 + 95 + 25 = 164. Throughout the length of the line, the glands are crowded, so that the anterior and posterior aspects are flattened. They are not segmental. Dissection showed a total of 122 myotomes, excluding the myotomes modified in the head. These are arranged 31 + 64 + 27. In the middle of the abdominal region there are 15 glands in a distance equivalent to 10 myotomes.

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The lingual teeth have the usual arrangement, there being an anterior and posterior row on either side. The anterior series has eight teeth on each side; the first and second are fused to a common base. The second series has nine teeth on either side, the first and second also fused to a common base. The teeth are strongly compressed, with a sharp posterior edge, tapered to a point, and cream in colour. The branchial pouches are seven on either side, diminishing in size from the anterior to the posterior pouch, bluntly ovoidal in form, and connected by individual ducts to a short common branchial atrium receiving the oesophageo-cutaneous duct on the left side.

The larger specimen is a female containing elongate ova up to 4.5 cm. in length. The colour preserved in spirits, greyish-brown, darker above. The total length, 315 mm.; depth at middle of body, 18 mm.; from tip of rostrum to level of lower lip, 8 mm.; to level of branchial apertures, 77 mm.; to level of vent, 265 mm. This is designated as the holotype, and is held at the Department of Zoology. Victoria University College.

The smaller specimen has not been sexed. It measures approximately 163.0 mm. The tip of the rostrum to the branchial apertures is 40 mm. The first mucous pore is 12 mm. from the tip of the rostrum. Excepting in the smaller number of mucous pores (a total of 137), the external features are those of the larger specimen.