MUSEUM OF COMPAKATTVE ZOOLOGY. 23 



suggested for the order, and gives Pternodonta as a substitute. The 

 opinion advanced by me in regard to the propriety of placing the genus 

 nearer than any other of the recent sharks to the fishes, he accepts with- 

 out hesitation. He dissents emphatically, however, in regard to the 

 relations to extinct types, basing his objections on Dr. Traquair's dis- 

 covery of Ctenacanthus costellatus, in which he says the Doctor has 

 proved beyond a doubt that the Cladodont dentition and the ctenacan- 

 thoid spines coexisted in the same fish. Cladodus, he says, was obvi- 

 ously not at all related to Chlamydoselachus, and adds that it did not 

 have the essential dentition of that genus. Agreeing to some extent 

 with Cope, he asserts that Chlamydoselachus did have a representative 

 in the carboniferous genus Diplodus Agass. (Didymodus Cope), but 

 doubts that the two can be congeneric. In this letter the sharks are 

 arranged to include the new type. The arrangement given places 

 Hybodus, Cladodus, Ctenacanthus, etc., the Hybodontidse, in the Lipo- 

 spondyli ; and Chlamydoselachus and Didymodus, which he calls Chla- 

 mydoselachidse, in the Selachophichthyoidi. It is also suggested in the 

 note, that the Hybodonti may not have been Squali at all, but may be 

 more nearly related to the Holocephali, the primitive form from which 

 both diverged being theoretically like Ctenacanthus. 



The next publication on the subject was that of ]\[r. Cope in the 

 American Naturalist, April, 1884, p. 412 : — 



" Tlie Skull of a still living Shark of the Coal Measures. — The genus Didymo- 

 dus is a well-known form of Elasmobranchi of the Coal Measures, and I have 

 reported it as occurring also in the Permian. Mr. S. Garman has recently 

 published an account of a shark supposed to have been taken off the coast of 

 Japan, which he names CJilaviydoselachus anguineus, referring it to a new genus 

 and family. He figures the teeth, and these are, as I have pointed out, identi- 

 cal with those of the genus above-named. The species should then he called 

 Didymodus anguineus." 



After disposing of the genus Chlamydoselachus, this writer in the same 

 article proceeds to give a description of the skull and teeth of Didymo- 

 dus, which we take occasion to quote and discuss below, p. 28. 



Science of April 11 contains a letter from Professor Gill on "The 

 Eolations of Didymodus or Diplodus," in which, commenting on Cope's 

 note, he says : — 



"A resume oi Professor Cope's observations has just appeared, as promised, 

 in the American Naturalist for April (XVIIl. 412), and we are therefore in 

 a position to test his utterances. Notwithstanding the reverence and confidence 

 that I have expressed, I can but think now that for once Professor Cope has 



