MUSEUM OF COMPARATIVE ZOOLOGY. 31 



come from an intei'mediate between the Hybodonti and the Cladodonti, 

 a branch from the latter. From the Cladodonti we have the Notidani 

 and a portion of the Lamnse. Probably the line leading to the Notida- 

 nidse had separated from that of Cladodus before the Carboniferous; 

 but that it belongs to that line appears probable, in view of the affini- 

 ties to Chlamydoselachus, and the presence in the upper jaws of Cla- 

 dodont teeth and intermediates between them and the most modified 

 ones in the lower jaws. 



A great diff"erence between the teeth of Cladodonts and others is to 

 be seen in the bases. In Cladodus the teeth are braced posteriorly, by 

 a backward prolongation of the base, most often extended under the fol- 

 lowing tooth in the same row. As soon as teeth of this character emerge 

 from the thecal fold under which they are formed, they come into func- 

 tion, on the inner side of the jaws, and continue in use even after passing 

 to the outer side. 



Another plan is to be seen in many of the blade- or chisel-like cutting 

 teeth, Scymnus and others, or in the broad teeth of the JSTotidanidse. 

 Here the bracing is done by a forward prolongation of the base, the 

 teeth coming into function only after the extension has passed to the 

 outer side of the jaws. In Pristicladodus of McCoy {P. dentatus), and 

 its possible descendants, Carcharodon and Carcharias, or in Lamna, there 

 is also a slight backward extension, which has the effect of bringing the 

 tooth into function sooner by raising the apices of the cusps. 



In Hybodus pi'oper, the teeth are intermediates in character of base, 

 being braced neither forward nor backward, or but slightly in either or 

 both directions. Such teeth come into use somewhat early, and become 

 useless soon after turning to the outer side of the jaw. Between the 

 Cladodonts and the Hybodonts there are many intermediates, some of 

 which are evidently out of place as now situated in Hybodus. 



Before the discovery of Chlamydoselachus it was supposed the line of 

 the Cladodonti had become extinct in the carboniferous. Now, with 

 Cladodus at the farther end, their line is extended from the Sabcarbon- 

 iferous to the present ; or, taking Pternodus (Pristicladodus St. John 

 and Worthen, not of McCoy, type P. springeri St. J. & W.) — a genus 

 allied to both of the preceding genera — as our point of departure, it 

 extends from the Middle Devonian. 



How much we are justified in treating Chlamydoselachus as a per- 

 sistent type, palseontological research will ultimately determine. The 

 reasons for so doing are found in the identity of dental characteristics 

 of Cladodus and its recent representative, and in the possession of char- 



