24 BULLETIN OF THE 



been too hasty, and tripped. I am convinced, not only that Didymodus has no 

 generic nor even family relations with Chlamydoselachus, but that it represents 

 even a different order." 



This is followed by the history of Diplodus as worked out by Kner in 

 1867, with the addition of a more recent notice, the substitution of the 

 name Didymodus. The letter contains also expressions of doubt in 

 regard to resemblance between Thrinacodus and Chlamydoselachus. 



From a letter in Science of May 30, by Mr. Cope, in reply to the 

 foregoing, the following quotation is taken. The title of the letter is 

 "Pleuracanthus and Didymodus." After stating Gill's position the 

 author remarks : — 



" L There is no generic difference to be detected, in my opinion, between 

 the teeth which are typical of Diplodus Agas. and Thrinacodus St. J. & W. 

 and the recent Chlamydoselachus. Differences there are, but apparently not of 

 generic value 



"3. Diplodus being regarded as a synonym of Pleuracanthus, it follows that 

 Chlamydoselachus Garm. is distinct on account of the different structure of the 

 dorsal fin, which is single and elongate in Pleuracanthus, according to Geinitz 

 and Kner. The presence of the nuchal spine is also probably a character of 

 distinction, although we do not yet know whether such a spine is concealed in 



Chlamydoselachus or not I suspect that the skulls I describe represent a 



different genus from Pleuracanthus proper. This genus will not differ from 

 Chlamydoselachus Garm. so far as we know the latter ; but the button indicates 

 another species 



" 5. Of course a study of the anatomy of Chlamydoselachus, which I hope 

 ]\rr. Garman may soon give us, may reveal differences between that genus and 

 Didymodus ; but of these we know nothing as yet. 



The next publication on the subject is that of Mr. Cope in his Palse- 

 ontological Bulletin, No. 38, printed July 1, "On the Structure of the 

 Skull in the Elasmobranch Genus Didymodus." This bulletin consists 

 of pages 503 to 590 of the Proceedings of the American Philosophical 

 Society of Philadelphia for 1884. In the article there are several para- 

 graphs relating, with more or less directness, to the frilled shark, the 

 substance of the most of which has been indicated above. The following 

 forms the opening paragraph of the paper : — 



" The genus Diplodus was described by Agassiz from specimens of teeth 

 from the European Coal Measures. In America, Newberry and Worthen have 

 described four species from the Carboniferous of Illinois and Ohio ; and I have 

 reported two species from the Permian beds of Illinois and Texas. Recently 

 Mr. Samuel Garman has described a shark, said to have been taken in the 

 Japanese seas, under the name of Chlamydoselachus anguineus, whose teeth, as 



