DAVIS: NOTE OX CHLAMYDOSELxiCHUS. 
101 
This communication from Professor Gill was followed by 
another from the pen of Professor Cope, in Science, of May 30th, 
1884, in which he acknowledges that he stands corrected by Professor 
Gill's superior knowledge of the literature of the subject ; the genus 
Diplodus he regards as a synonym of Pleuracantlms, but as Pleura- 
canthus has a nucal spine along with tricuspidate teeth and the 
specimens which he regarded as the type of the genus Didymodus 
have not hithei'to been proved to have spines, though the spines of 
Pleuracanthus have been found separately in the same beds, Professor 
Cope thinks that the identity of Didymodus with Pleuracanthus may 
now be questioned. He also states that there is no generic difference 
to be detected between the teeth which are typical of Diplodus, 
Agassiz, Trinacodus, St. John and AVnrthen, and the recent Chlaniy- 
doselachus : there arc differences but not of generic value. But, 
as Diplodus must be regarded as a synonyme of Pleuracanthus, it 
follows that Chlamydoselachns, Garm., is distinct on account of the 
different structure of the dorsal fin, which is single and elongate in 
Pleuracanthus : the presence of the nucal spine in Pleuracanthus is 
also, probably a character of distinction. Professor Cope further 
suspects that the skulls of Didymodus he has described represent a 
different genus from Pleuracanthus proper, and that so far as we 
know Chlamydoselachns, it will not differ from Didymodus, though 
a study of the anatomy of the former may reveal differences between 
that genus and Didymodus. These views were reiterated by 
Professor Cope in his paper on the structure of the skull in the 
Elasmobranch genus Didymodus, published in July in the Proceedings 
of the American Philosophical Society of Philadelphia. 
The discussion on the palpeontological affinities of this remark- 
able fish appear to have reached a climax at the meeting of the 
American Association for the Advancement of Science, at 
Philadelphia, in September, 1884, when Mr. S. Garman read a paper 
on his discovery of Chlamydoselachns, and after discussing its 
relationship. Professor Cope abandoned his position concerning the 
afiSnities of Didymodus and the recent fish, and agreed that judging 
from the teeth alone, its nearest known allies were the Cladodonts 
of the Carboniferous and Devonian rocks, and Professor Gill writing 
