MUSEUM OF COMPAEATIVE ZOOLOGY. 25 



represented, do not differ generically from those of Diplodus. This is an inter- 

 esting discovery, indicating that this genus, and not Ceratodus, is the oldest 

 type of vertebrate now known in the living state." 



Near the end of the paper the quotations given above, under 1, 3, and 

 5, are repeated. 



In a letter to Science of November 28, on " The oldest living Type of 

 Vertebrates," I called attention to the fact that, when my paper on the 

 recent discovery was read before the American Association for the Ad- 

 vancement of Science, Philadelphia meeting, Sept. 4, 1884, Professor 

 Cope abandoned his position concerning the affinities of Didymodus 

 and Chlamydoselachus, and agreed with me in the conclusions that the 

 two genera were very different, and that, from all the data we had for 

 comparison, the nearest known allies of Chlamydoselachus were Clado- 

 donts of the Subcarboniferous and Middle Devonian. The same letter 

 pointed out certain necessary changes in the classification, such as the 

 separation of the Cladodonti from the Hybodonti, and their inclusion, 

 with Chlamydoselachus, in a separate group characterized by "vertebrae 

 partially or imperfectly developed, a persistent notochord, and teeth 

 with broad backward-expanded bases." The communication also ad- 

 vanced the idea that the connection of the trabecular process of the 

 pterygo-quadrate with the skull, in such genera as Chlamydoselachus, 

 Rhina, and others, though it is of secondary origin, is none the less a 

 true articulation. 



Up to date, the last word in the discussion is that of Professor Gill, in 

 reply to my note of November 28 ; it is in the same journal, issue of 

 December 12, and has the same heading. The several statements con- 

 tained in it, in regard to which we should disagree most seriously, are 

 the following : — 



" The differences between us now are fictitious rather than real, or better, 

 perhaps, they are chiefly differences of expression." " The palato-pterygoid 

 not articulated with the skull is a true character of the typical sharks and 

 Rhinse." " I must dissent from the opinion that the Cladodontidse are related 

 to the Chlamj^doselachidae rather than the Hybodontidoe." 



The characters of genus and fiimily assigned in the preliminary de- 

 scription were essentially as given below. 



ChlamydoselacMdcB. 



Body elongate, slender. Head broad, depressed. Eyes lateral, with- 

 out a nictitating membrane. Nasal cavity separate from that of the 



