The Anatomy of Chlamydoselachus 349 



Among fossil forms assigned to other genera, teeth more or less resembling those 

 of Chlamydoselachus anguineus are found in Cladoselache (Text'figure 17), in Cladodus 

 (Text'figure 18), in Ctenacanthus (Text'figure 19), and in Hyhodus (Text'figure 20). In 

 each of these fossil sharks the teeth vary in form, but those represented in the figures 

 may be regarded as typical. In all these teeth the cusps are conical, and the central 

 cusp is by far the most prominent. In Hyhodus the lateral cusps (3 or 4 on each side) 

 become smaller in proportion to their distance from the central cusp. In Cladodus, 

 Ctenacayithus and Cladoselache there are two cusps on each side of the central cusp, and 

 the marginal cusps are larger than the intermediate cusps. In Cladoselache the inter' 

 mediate cusps are very small, as in the frilled shark. In Hyhodus and in Cladodus most 

 of the cusps are recurved at the tip. In Cteyiacanthus and in Cladoselache the cusps are 

 more slender and appear practically straight, though Dean (1909) states that in Clado- 

 selache clar}{i there is a slight sigmoid flexure of the cusps. Of the four forms considered, 

 Cladoselache possesses the sharpest cusps. In this, as in many other respects, the teeth 

 of Cladoselache most nearly resemble those of the frilled shark, but in this connection 

 I quote the following from Dean, 1909, p. 253: 



When teeth of the type of Cladodus were discovered in different horizons from the 

 Devonian well into the Mesozoic, it was naturally concluded that the sharks themselves would 

 be found to correspond closely — to belong if not to the same genus at least to the same family. 

 When, however, associated remains of the earlier forms were discovered, it became clear 

 that these sharks were by no means closely allied. Instead of being proven to be cestracionts, 

 one type of ^'Cladodus" {Cladoselache \epleri, C. fyleri; Upper Devonian), was found to be 

 spineless, and quite different in essential structures from the modern cestraciont: another 

 type of ''Cladodus" Symmorium Cope (Coal Measures), was then shown to be unlike both 

 Cestracion and Cladoselache; and still another, ''Cladodus" neilsoni, was demonstrated by 

 Traquair to be quite different in fin characters from all the rest. And now a fourth cladodont, 

 Ctenacanthus, is found notably discrepant. It is, then, only the mesozoic group of "clado- 

 donts" typified by Hyhodus which remains faithful to our preconceived notions as to what 

 kind of a shark a cladodont tooth should predicate. The fact of the matter is that the clado- 

 dont type of tooth is as ancient as it has been useful in the subclass Elasmobranchii, and that 

 it has appeared in many different lines, either as an heirloom from primitive sharks, or, less 

 probably, as an independent acquisition. Certain it is that it appears with little variation 

 in as many as seven families of sharks, and in at least three distinct orders. 



When teeth are highly differentiated, resemblances amounting almost to identity 

 (as between Chlamydoselachus anguineus and C. laivleyi) are probably significant. On 

 the other hand, among living fishes we find instances where members of the same family 

 have widely different teeth. On a priori grounds it seems likely that, where cusps are 

 numerous and close together, development may proceed by the eHmination of some of the 

 cusps in order that the others may be better nourished; or, putting the matter in another 

 way, some cusps may develop at the expense of the others. It seems probable that, in 

 the long lapse of time, teeth like those of Chlamydoselachus anguineus could have evolved 

 out of rather irregular and rudimentary structures, like the teeth of Hyhodus reticulatus 



