"the Anatomy of Chlamydoselachus 339 



From its primitive position, the mouth may be displaced either ventrally or terminal' 

 ly. In elasmobranchs it is usually displaced ventrally by the thickening and forward 

 elongation of the cranium to form a rostrum. In other words, when the cranium becomes 

 extended anteriorly, the mouth of necessity becomes ventral. This may occur regardless 

 of the si2;e of the mouth. In the basking shark, Cetorhinus, the mouth is very large but is 

 nevertheless ventral because of the elongate snout. In the sawfishes the prolongation of 

 the rostrum is carried to an extreme that makes the mouth decidedly ventral. In teleosts 

 the mouth tends more often to become terminal, though in some forms, as in the fresh' 

 water suckers, it is brought into a ventral position by an extensive development of the 

 related soft parts. 



I conclude that, in connection with its enormous enlargement, the mouth of Chlamy^ 

 doselachus has departed only slightly from the primitive orientation, and that this de- 

 parture has been in the direction of a more nearly terminal position. The anatomical 

 basis for this condition is described more fully in the section on the skull. The position 

 of the mouth is decidedly more primitive in Chlamydoselachus than it is in most elasmo' 

 branchs; it shows a closer parallel with the condition usually found in teleosts. But 

 there is substantial evidence, which cannot be considered here, indicating that the line 

 of cleavage between elasmobranchs and teleostomes extends back to forms more general' 

 izied than any living fish. 



GILL-COVERS AND SPIRACLES 



The presence, in Chlamydoselachus, of a sixth pair of gill-slits has usually been 

 accounted a primitive character of considerable phylogenetic importance, linking Chlamy- 

 doselachus with the notidanids. But Pliotrema, a sawfish, has six pairs of gill'slits (Regan, 

 1906.1), differing in this respect from other sawfishes. While there is abundant ground for 

 the conviction that Chlamydoselachus is related to the notidanids, one must not lean too 

 heavily on the evidence afforded by the number of gill-slits. "In the existing elasmo- 

 branchs the normal number of gills is five and it may well be suspected that the six or 

 seven gill-slits of the notidanids and the six of Pliotrema represent a secondary increase 

 in number" (Gregory, 1933, p. 424). 



In Chlamydoselachus, the unusually well developed first pair of gill-covers (Text- 

 figure 4), continuous as the gular fold across the mid'ventral line, simulates an operculum 

 such as is found in bony fishes. Garman (1884.2) suggested that this operculum-like 

 fold or collar of Chlamydoselachus is a character indicating that the frilled shark lies near 



Text-figure 4. 

 A side view of the head of Chlamydoselachus to 

 show the position of the mouth, the length of the 

 lower jaw, the position of the nostril and of the 

 eye, and the position and form of the gill-covers; 

 about one-fourth natural si2;e. 

 After Garman, 1885.2, pi. I. 



