338 Bashford Dean Memorial Volume 



More than in most sharks, the head of Chlamydoselachus, though not its body, is 

 decidedly flattened in a dorsoventral direction when the jaws are closed. This, together 

 with the fact that the creature is usually taken at great depths, suggests that the frilled 

 shark is, at least partly, a bottom-dwelling form. We need not, however, conclude that 

 the flattening of the head tends to remove Chlamydoselachus from the category of archaic 

 fishes. "For various reasons it seems likely that the primitive chordates were not swift' 

 swimming, pelagic types but partly depressed, partly bottom-living forms" (Gregory, 

 1933, p. 101). 



Among living sharks the notidanid Heptanchus maculatus (Text-figure 3), though 

 stouter-bodied than Chlamydoselachus, presents the greatest similarity in general form, 

 position and shape of the fins, and in the shape of the tail. Throughout the present 

 article I have made many comparisons between Chlamydoselachus and Heptanchus. Dean 

 (1895) stated that ^'Heptayichus, of all living sharks, inherits possibly to the greatest 

 extent the features of its remote ancestors." This is doubtless still a fair generalization 

 when one considers only the external characters, but in many, perhaps most, of the 

 internal structures described in the present article, Chlamydoselachus is less specialised 

 than Heptanchus. 



POSITION OF THE MOUTH 



In Chlamydoselachus the mouth is sub-terminal (Text-figure 4, after Garman), but 

 it approaches a terminal position to a degree found in no other shark, so far as I know, 

 save only Rhineodon, the whale-shark. Sharks are pre-eminently surface-feeding forms, 

 but the mouth is usually ventral. In skates and rays, which are bottom-feeding fishes, the 

 mouth is decidedly ventral. In teleosts, with the exception of a few bottom-feeding 

 forms, the mouth is terminal or subterminal. Thus in fishes the position of the mouth is 

 decidedly variable. In linking the great groups of fishes, to assign phylogenetic value to 

 such a character is hazardous. One cannot fail to note the resemblance, in the position of 

 the mouth, between Chlamydoselachus and the teleosts, but their real relationship must 

 be decided on the basis of more stable characters. Nevertheless, it may be pertinent to 

 inquire, what is the primitive position of the mouth in the vertebrates? 



Since in vertebrate embryos the mouth is ventrally situated, one might infer that 

 this position is primitive for vertebrates. This inference is not supported by all the 

 facts of development. The ventral position of the mouth of a vertebrate embryo is due, 

 in part to a precocious enlargement of the anterior end of the brain, in part to the cephalic 

 and cervical flexures which, in later development, tend to straighten out. If we consider 

 only adult structures and accept the time-honored theory that the jaws represent a modi- 

 fied gill-arch, then the mouth is formed on the morphologically anterior side of this gill- 

 arch. In its primitive position the mouth would naturally open forward, though situated 

 at a lower level than the cranium and to this extent not fully terminal. The vertebrate 

 mouth is, primarily, anteroventral or subterminal. 



