EXTERNAL FORM 



EXTERNAL FORM OF HEPTANCHUS MACULATUS 



Hepianchus maculatus (fig. 13), in general shape, lacks the grace character- 

 istic of many of the more active and predacious sharks. This is due in part to 

 a relatively heavy head; in part it results from the unusual dimensions of the 

 tail. In these features all heptanchid sharks (figs. 12 to 14) are similar. In 

 Hexanchus, a close relative of Heptanchiis, the body is still more ungainly. 

 This is readily apparent when a specimen is brought to shore, for on land the 

 body and head are so heavy that they flatten out and become distorted. Not- 

 withstanding this lack of grace in the heptanchid sharks, I agree with Dean 

 (1895) that "Hepianchus, of all living sharks, inherits possibly to the greatest 

 extent the features of its remote ancestors." 



The most superficial feature by which Hepfanchus may be recognized is 

 the number of its gill clefts (cL, fig. 15) . In fact it was from this characteristic 

 that Hepianchus received its name.^ Anteriorly the clefts are of large size, but 

 posteriorly they decrease so that the last, which lies just in front of the pec- 

 toral fin, is about half the height of the first. Anterior to the first cleft and 

 somewhat dorsally is the modified cleft or spiracle (sp., fig. 15) which in the 

 adult is relatiA'ely diminutive in size. 



Other superficial characteristics helpful in distinguishing Hepianchus from 

 most other sharks are the position of the nasal apertures and the shape of the 

 mouth. The nasal apertures are more nearly terminal than is usual in the 

 Elasmobranchs. They appear on the broad snout as relatively small crescents, 

 each of which is separated into a dorsal and a ventral part by an overlapping 

 median flap. This flap in fact forms of the nasal cup a tube which provides a 

 passageway for the water current over the olfactory organ. The mouth is of 

 unusual size (figs. 15 and 119) . In side view it appears as a deep horizontal slit 

 extending back as far as the segment of the spiracle, and hence cleaving the 

 anterior region almost into dorsal and ventral halves. From this type of cleav- 

 age there results an exceptionally heavy lower jaw which gives to this form 

 much of its grotesque appearance. 



At the side of the head is the eye, which has an orbit of relatively long hori- 

 zontal axis. The eye is shielded by an upper and a lower membranous lid, but it 

 is unprovided with the so-called nictitating membrane or third eyelid charac- 

 teristic of some of the Elasmobranchs. 



This shark also possesses, in common with other Elasmobranchs, the small 

 apertures of the endolymphatic ducts which lead to the ear (see p. 42, fig. 45, 

 e.cl.). These apertures are near the middorsal line and slightly in front of a 

 plane taken through the spiracles. 



1 €7rra, Seven; a7xw, with reference to compressed gill clefts. 



[5] 



