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 :— 
“J, 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 
Mr. 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 Pale- 
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 
