DAVIS : KOTE ON CHLAMYDOSELACHUS. 
107 
valves. Behind the auricle and above and behind the ventricle, lies 
the sinus which has a capacity that nearly equals the bulk of the 
ventricle. In sharks the bulbus arteriosus is short and the rows of 
valves number from two to five. 
The internal organs were for the most part torn away and lost, 
but sufficient of the intestine remains to show that it possessed a 
spiral valve and a caecal pouch behind the valve. 
The genus Chlamydoselachus differs so materially not only in 
its dentition, but also in its general form and structure from that of 
Pleuracanthus, Ag. (Xenacanthus, Beyrick), that there can be little 
hesitation in deciding that no near relationship exists between the 
two. Pleuracanthus had a broad, depressed head, large in proportion 
to the size of the body, rather short but very wide gape, the anterior 
portion of the jaws forming the extremity of the head. The gill 
arches were four or five in number. The well-known spine was 
attached immediately behind the occupital region, and was one-fourth 
or one-fifth the entire length of the fish. The pectoral fins were 
firmly attached and very large, the ventrals are somewhat less. 
The dorsal fin extended nearly the w^hole length of the back, 
encircled the tail, and was continued along the ventral surface : it 
was supported by ossified sphious and interspinous processes. The 
vertebrae and crainial supports were cartilaginous, the latter with 
innumerable osseous centres. My views with respect to the classifi- 
catory position and natural affinities of the genus Pleuracanthus have 
been expressed in papers published in the Annals and Magazine of 
Natural History (Ser. V., Vol. V., p. 349), and in the Quarterly 
Journal of the Geological Society (Vol. XXXVI., p. 321), and it 
need only be further remarked that the comparatively short, tapering 
body, large broad head with occupital spine, the number of its 
branchial arches, and the general character of the fins offer a striking 
contrast to the elongate form, small head, six or seven branchial 
arches and scanty fins of Chlamydoselachus. The resemblance of 
the teeth at first insisted upon by Professor Cope is only a superficial 
one and rests simply on the accidental circumstance of each having 
three denticles. 
In the Paloeozoic strata of Britain, with the exception of one 
