28 BULLETIN OF THE 
the base are not seen, and there seem to be no rudimentary denticles. 
What is exposed favors to some degree the conclusion of its discoverer ; 
but we do not know that the tooth has the lateral cusps, the backward- 
expanded base, or the rudimentary denticles of the typical species of 
Cladodus. 
The probability is that Cladodus, known only from dentition, — which 
would include teeth of Rhina or the upper front teeth of Heptabranchias, 
— contains species that, if living to-day, would be distributed among a 
number of genera, or even among different families. This being the case, 
it might not be out of the way for Dr. Traquair to claim that Ctenacan- 
thus — with a short body, two dorsals each with a spine, a mouth simi- 
lar to that of Heterodontus, and teeth with one cusp and no buttons — 
is a Cladodont; while Chlamydoselachus — with elongate body, a spine- 
less dorsal, an ophidian mouth, and teeth with lateral cusps and buttons 
— is claimed to represent Cladodus itself. 
Chlamydoselachus, however, has not been an undisputed Cladodont. 
In several publications Professor Cope has asserted its identity with 
Diplodus of Agassiz (renamed Didymodus by Cope, but later admitted to 
be identical with Xenacanthus). As he has since abandoned his posi- 
tion, it would be unnecessary to consider the subject further, if it were 
not that he has made no publication of his change of opinion, and that 
matter in one or two of the communications may be used to throw 
light on the affinities of Chlamydoselachus and allied sharks. From 
the American Naturalist of April, 1884, page 412, we quote the fol- 
lowing description of skulls said by its writer to belong to the genus 
Diplodus Ag., renamed Didymodus, in which Mr. Cope claimed the 
frilled shark must be placed (see page 22). 
“The palatopterygoid arch is suspended to the postorbital process of the cra- 
nium, as in the existing Hexanchide. The genus would then be referred to the 
suborder Opistharthri of Gill, but for the following peculiarities : The skull is 
segmented, so that cartilage-frontals, parietals, and occipitals can be distin- 
euished, together with an element which has the position of the intercalare. 
The occipital supports a large vertebral cotylus. There are membrane bones 
extending from the nose over the orbits, which are either supraorbitals or 
frontals. The tissue of the bones is granular, which leads to the belief that the 
granular ossification which covers the chondrocranium in recent sharks, pene- 
trated the entire chondrocranium in this genus. Hence the basicranial axis 
consists of the sphenoid and presphenoid bones. One at least of the nares is 
on the superior face of the muzzle. The frontal cartilage-bones are elevated 
and fissured at the posterior extremity, each apex projecting freely upwards and 
backwards, presenting a certain resemblance to the structure seen in the Lepi- 
