30 



BULLETIN Oi* THE 



The name Didymodus having been proposed as a substitute for 

 Diplodus Ag., it passes out of use as a synonym for Xenacanthus Beyr. 

 Such being the case, it is not available as a name for a new genus. 

 Professor Cope suspects that the skull he has described represents a 

 different genus from Pleuracaathus proper (Xenacanthus Beyr.), which 

 " will not differ from Chlamydoselachus Garm." As it does differ very 

 decidedly from the latter genus, confusiou will be obviated by allow- 

 ing the name Didymodus to remain a synonym, and employing a new 

 name, say Diacranodus, for the new genus. The species will then stand 

 D. compressus and D. platypternus. The genus may be distinguished 

 by the attachment of the pterygo-quadrate to the post-orbital process of 

 the cranium, and by the teeth : cusps two, diverging, subconical, slender, 

 and separated by a median rudimentary denticle or button on the base; 

 bases extended backward, thinner and rounded posteriorly. 



By reference to page 25, it will be seen that, while on some points we 

 accord well, in others there is considerable variance between Professor 

 Gill's views and my own. Against his assertion that Cladodus is not at 

 all related to Chlamydoselachus, and that it did not have the essential 

 dentition of the latter, I am compelled to maintain that these genera 

 have essentially the same deutition, that they are very closely related, 

 and that, in fact, the recent genus is the living representative of the 

 fossil Cladodus. As the letters and replies noted in the history, pages 

 22-25, discuss the question, it is not necessary to give it further atten- 

 tion here. In regard to the recent affinities our diffei-ences are slight. 

 The arrangement given below expresses the conclusions reached as nearly, 

 perhaps, as any at present made. A comparison with Professor Gill's 

 letter in Science, March 21, 1884, will show the extent of our agree- 

 ments and differences. 



Hyhodonti: Hybodus and allies. 



Cladodonti: Cladodus and allies ; 

 Chlamydoselachus. 



Notidani: Hexanchus and Hepta- 

 branchias. 



Heterodonti: Heterodontus (Ces- 

 tracion Cuv.). 



Lavmoe: Most living sharks. 

 \.Ehinoe (Squatinse). 



fSELACHII^ 



Elasmobranchii ■ 



Galei 



Batoidei. 



IHOLOCEPHALI. 



Without resorting to the "genealogical tree," a scheme that shall not 

 separate groups that belong to the same line of descent is difficult to 

 an-ange. In the above the Hybodonti were probably the progenitors of 

 the Heterodonti and a portion of the Lamnse. The Ehinse may have 



