Cope.] OOO [March 7, 



In 1857 Sir Philip de Malpas Gray Egerton (Ann. and Mag. Nat. Hist., 

 xx, 423), contended that the spines of Pleuracanthus belonged to the 

 same fish as the Diploclus teeth, and that Xenacanthus was likewise refer- 

 able to the same type. 



In 1867 Prof. Kner (Sitzb. k. Akad. wiss. Wien, lv, 540-584), published 

 a memoir, illustrated by ten plates, in which he proved that Diplodus and 

 Xenacanthus were generically identical. 



In 1875 Messrs. St. John and Worthen proposed the genus Thrinacodus 

 for the Diplodus incurvus and D. duplicatus of Newberry and Worthen and 

 the T. nanus St. J. and W., from Illinois. 



In 1883, in the Proceedings of the Philadelphia Academy (p. 108), I 

 proposed the name Didymodus for the Diplodus compressus Newberry. 



In Science for 1884, p. 274 (March 7th), I called attention to the close re- 

 semblance of the teeth of this genus to those of the recent shark, called by 

 Grarman Chlamydoselaclms, and expressed my belief in the identity of the 

 two genera. 



In the American Naturalist for April, 1884, p. 413, I gave a brief ab- 

 stract of the characters of the skull of Didymodus, and proposed to regard 

 it as the type of a new order to be called the Ichthyotomi. 



In Science, 1884, p. 429 (April 11), Prof. Gill objects to the identification 

 of the genera Didymodus and Chlamydoselaclms ; on the ground of the dif- 

 ferent forms of the teeth. He states that he doubts the pertinence of the 

 two genera to the same order. He points out that the oldest name for Dip- 

 loclus Ag. is Pleuracanthus Ag., and that the order Ichthyotomi had been 

 already defined and named by Liitken, with the name Xenacanthini. 



On these various propositions the following remarks may be made. 



(1.) There is no generic difference to be detected, in my opinion, be- 

 tween the teeth which are typical of Diplodus Agass. and Thrinacodus St. 

 J. and W. and the recent Chlamydoselachus. Differences there are, but 

 apparently not of generic value. The identification of the recent and ex- 

 tinct genera rests, as far as this point goes, on the same basis as that of the 

 recent and extinct Ceratodus. 



(2.) At the time of my proposal of the name Didymodus, I was not con- 

 vinced that fishes of this type bore the spines referred to the genus Pleura- 

 canthus Ag. None of the authors cited figure any specimens which pre- 

 sent both tricuspidate teeth and a nuchal spine. None of my ten speci- 

 mens possess a spine. However, Kner describes two specimens as exhibit- 

 ing both tricuspidate teeth and a spine, and Sir P. Egerton's statements 

 (J,, c), on this point are positive. So we must regard Pleuracanthus as the 

 name of this genus, with Diplodus as a synonym. 



(?.) Diplodus being regarded as a synomym of Pleuracanthus, it follows 

 that Chlamydoselachus Garm is distinct, on account of the different struc- 

 ture of the dorsal fin, which is single and elongate in Pleuracanthus, ac- 

 cording to Geinitz and Kner. The presence of the nuchal spine in Pleura- 

 canthus is also probably a character of distinction, although we do not yet 

 know whether such a spine is concealed in Chlamydoselachus or not. 



