MUSEUM OF COMPARATIVE 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 Hybodontide, in the Lipo- 
spondyli; and Chlamydoselachus and Didymodus, which he calls Chla- 
mydoselachide, 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 Mr. Cope in the 
American Naturalist, April, 1884, p. 412 :— 
“ The 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 Chlamydoselachus 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 be 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 
Relations of Didymodus or Diplodus,” in which, commenting on Cope’s 
note, he says : — 
“A résumé of Professor Cope’s observations has just appeared, as promised, 
in the American Naturalist for April (XVIII. 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 
