HISTORICAL REVIEW 5 



In the "Proceedings of the Philadelphia Academy of Natural Science" 

 (33), Cope substituted the name Didymod-us for Diplodns, as the latter name 

 was preoccupied, and named several new fishes: Thoracodiis emydinus, 

 Ctenodus heteroloplms, and Ctenodus vabasensis. 



Following this paper came one in the "Proceedings of the American Phi- 

 losophical Society" (34), in which Cope described and figured the skull of 

 the genus. This paper was preceded by two brief notes in the "American 

 Naturalist" giving preliminary accounts of the skull (35, 36). The use 

 of the name Didymodus by Cope provoked a discussion with Dr. Gill in the 

 columns of "Science" (vol. iii, pp. 275, 429, 645), but Cope did not alter 

 the name. 



In the first of these papers Cope proposed a new order of the Elasmo- 

 branchii to contain Didymodus, which he called Icthyotorm and defined as 

 follows : 



"A basioccipital bone and condyle. Occipital, {V) pterotic, and frontal 

 bones distinct. Supraorbital (or nasal) bones present." 



The remaining members of the Elasmobranchii were distinguished as a 

 separate group by the want of these characters and called by the old name 

 Selachii. 



Garman engaged in a discussion with Cope and published several short 

 papers in "Science" during 1884 and 1885, protesting against Cope's propo- 

 sition that Didymodus was identical with or closely related to the Chlamy- 

 doselachi. The most important and culminating paper was printed in the 

 "Bulletin of the Museum of Comparative Zoology" in 1885 (53) In this 

 he insisted that the two forms are not related and proposed a new name 

 Diacranodus for the Permian shark. He defines his genus as "distinguished 

 by the attachment of the pterygoquadrate to the postorbital process of the 

 cranium, and by the teeth; cusps two, diverging, subconical, slender, and 

 separated by a median rudimentary denticle or button on the base; bases 

 extending backward, thinner and rounded posteriorly." 



Cope replied to this in the "American Naturalist," criticizing Garman 

 and retaining the name Didymodtis (40). 



In the "American Naturalist, " Jan. 1884, Cope published a semi-popular 

 account of "The Batrachla of the Permian Period of North America" (37) ; in 

 this he gave a genealogical table of the class Batrachia and an analysis of 

 the characters of the Permian forms as follows: 



"i. Supraoccipital, intercalary (tabulate) and supratemporal (squamosal) 



bones present. Propodial bones distinct. 

 Vertebral centra, including the atlas, segmented, one set of segments 



together supporting one arch Rhachitomi 



Vertebrae segmented, the superior and inferior segments each complete, 



forming two centra to each arch Embolomeri 



Vertebral centra, including atlas, not segmented; one to each arch. . . .Stegocephali 



"As regards the extinct orders, the primitive type is evidently the Rha- 

 chitomi whose vertebral column displays an arrest of characters which are 

 transitional in the higher Vertebrata. From this group the orders Embo- 



