110 
DAVIS: NOTE ON CHLAilYD OSELACHUS. 
base in height, with two or more lateral denticles diminishing in size 
as they recede from the central one. They approach in form more 
nearly to the Hybodus of the Lias than to the Cladodus of the 
Mountain Limestone. 
After the deposition of the Coal Measures the place of the 
Cladodont is taken by the Hybodonts of the secondary rocks. 
Cladodus so far as we know became extinct. 
A consideration of the foregoing observations shows that with 
the exception of C. costellatus, Traq., there is nothing definitely known 
as to the structure of the genus Ctenacauthus. Dr. Traquair's species 
demonstrates clearly the form of the fish which carried the Ctenacan- 
thoid spines, but unfortunately the evidence is not so clear with regard 
to the teeth. The tooth exhibited on the specimen may be a Cladodus, 
but the imperfection of its somewhat contracted base and the absence 
of all, except the central cusp, leaves room to doubt its identity. At 
the same time in reviewing the fossil teeth of the lower limestone, 
it must be admitted that there is probably no other genus which 
possesses a denticular arrangement similar to C. costellatus, Traq., and 
this forms an argument in favour of the opinion of Dr. Traquair. 
There can be little doubt, however, that the light thrown by this 
specimen on the external form of the Ctenacanthoid fishes proves 
their close relationship with the Hybodonts of the secondary 
formations. 
Mr. Garman defines the position of Chlamydoselachus in the 
system of recent sharks as near the genera Hexanchus and Hepta- 
branchias. The structure of the brain and its possession of six 
branchial appertures clearly remove it from all others, and as it 
differs more than they do from other sharks, it lies further from the 
main body of the Galei. The shape of the body, position of the 
mouth, articulatians of the jaws, dentition, squamation, lateral line, 
pelvis, tail and tropeic folds, furnish characters sufficient to establish 
the distinctness of both genus and family. Its rank is determined 
to be somewhat lower than i^otidanus by its unsegmented notochord, 
elongated bulbus and numerous cardiac valves, chondrification and 
other particulars, whilst certain cephalic peculiarities and the 
shagreen associate it remotely with Squatina, and its position falls 
