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The Blind Electric Rays of the Genus Typhlonarke (Torpedinidae)

Typhlonarke aysoni (Hamilton) 1902. (Pl. 1, Figs. A, B, G, I. Text Figs. B, E. F.)

Typhlonarke aysoni (Hamilton) 1902. (Pl. 1, Figs. A, B, G, I. Text Figs. B, E. F.)

Study material.—Male 354 mm. T.L., female 244 mm. T.L. Collected by trawl from Cook Strait area.

Description.—Body greatly depressed, almost circular. Skin smooth, lacking dermal denticles. Length of disc 1.4 in the total length. Greatest width of disc at level of the last gill-slit; 1.6 in the total length. Greatest depth of body at the pectoral sympysis, 8.0 in the total length. Disc very much thinner peripherally. Caudal peduncle stout and slightly compressed.

Distance from anterior edge of disc to spiracles slightly more than depth of the body. Spiracles irregularly oval, their long axes directed outwards anteriorly. Margins of the spiracles produced into a definite rim. Eyes not visible externally, though a white patch anterior to each spiracle indicates their approximate position. Eyes very small, lying 1.0 mm. to 2.0 mm. below the surface of the skin. Gill-slits small, crescentic. Distance from anterior edge of disc to first gill-slit. 4.5 in the total length. Distance between first and second slits slightly greater than that between fourth and fifth and less than that between second and third or third and fourth. Nostrils fairly closely set, their posterior valves expanded and curled round, forming a nearly complete tube. Anterior nasal valves confluent as a fleshy nasal flap expanded distally and secured between the nostrils by a thick median septum, so that its wider free end almost covers the mouth. The extension of the nostrils and nasal flap makes a distinct step in the profile. Mouth small, slightly curved, and deeply inset, almost hidden externally by the nasal flap and the greatly enlarged, deeply incised lower lips. Teeth arranged in mosaic, varying from 10/10 to 12/12 per row. The tooth plates very narrow, extending across the median part of the jaws with four to five rows of teeth functional. Teeth in the posterior rows each with a single median cusp, long and sharply pointed, directed posteriorly. Anterior teeth with cusps eroded; cusps almost lacking in the first row.

Dorsal fin originating above the posterior insertion of the pectoral fin. Rectangular in shape, its height 1.5 in its basis and 2.0 in the distance from anterior edge of disc to spiracles. Posterior edge rounded and extending behind the posterior insertion of the fin. Origin of caudal just anterior to the posterior edge of the dorsal, its height slightly less than its length, outline subcircular. Pectorals completely fused into the body, their margins smoothly curved. Paired electric organs present, reniform in shape, extending from midway between the anterior edge of disc and the eyes, to midway between the pectoral and pelvic symphyses. The greatest width of each organ equal to little more than one-fourth the width of the disc. Origin of pelvics not quite half of total length, from anterior edge of disc. Anterior portion of each pelvic free, and produced into a broad flat appendage which when appressed is directed obtusely to the long axis of the body. Width of the appendage about 2.5 in its length. Posterior portion of the pelvic fused with the pectoral as an extension of the disc without any notch between it and the pectoral, though there is a short thick web separating them. Lateral margin smoothly rounded and a slight notch at the posterior insertion. Gaspers small page 5 and somewhat flattened, not reaching to the posterior edge of the pelvics. Length of the claspers 1.8 in the length of appendage of the pelvic, and width 3.0 in the width of the same appendage. Cloacal aperture opposite the posterior insertion of the appendages of the pelvics.

Colour.—Upper surface brown, darker posteriorly. Under surface creamy-brown. Underside of pelvic fin appendage almost white.

Size.—Most specimens taken are under 400 mm. in total length, though Whitley (1940) states that Graham records one about 900 mm. wide, so that this must have exceeded 1100 mm. in total length.

Distribution.—Foveaux Strait; the coast of Otago; Cook Strait; 28 to 102 fathoms.

References and Synonymy.—Astrape aysoni Hamilton, A. (1902), Trans. N.Z. Inst., 34, p. 224, Pl. XII Fig. b (Pls. X, XI, and XII Fig. a are referable to T. tarakea). Typhlonarke aysoni: Waite, E. R. (1909), Rec. Cant. Mus., 1, No. 2, p. 146, Pl. XVIII; Phillipps, W. J. (1929), N.Z. J. Sci. & Tech., 11, No. 2, p. 100, Fig. 2; Whitley, G. P. (1940), Fishes of Australia, Part 1, p. 163, Fig. 184; Fowler, H. W. (1941), U.S. Nat. Mus. Bull., 100, vol. XIII, p. 353.