CRETACEOUS FISHES — DUNKLE 271 



from the "Benton" of Kansas is apparently known only by the 

 fragmentary type specimen. No differences can be observed between 

 the scales and vertebrae of this specimen and the corresponding 

 parts of Syllaemus and Leptichthys. Most authorities (for example, 

 Hay, 1929; Romer, 1945) continue to treat Syllaemus and Leptichthys 

 as distinct genera, but in addition to their similarity of scale and 

 vertebral structure, no major variation has been detected as yet 

 among available skulls of the two genera. Certainly the North 

 American materials which have been variously referred to either 

 Apsopelix, Syllaemus, or Leptichthys constitute a basically homo- 

 geneous group peculiar to a number of marine strata exposed in the 

 Western Interior, which in geologic range seem restricted to the 

 interval between either the late Cenomanian or early Turonian and 

 the Santonian stages of the upper Cretaceous. 



Relationship between the Apsopelix-Syllaemus-Leptichthys complex 

 of forms and the late upper Cretaceous herycinus species is 

 denoted by the common possession of distinctive modifications of 

 the bone pattern of the skull, and by general over-all similarities in 

 body habit, including the structure and relative position of fins. As 

 might be anticipated, on the other hand, certain regularly occurring 

 variations are to be observed. Of these, the proportions of the 

 parts of the visceral skeleton differ and these features may account 

 for the curious apposition of the mandibles commonly displayed by 

 specimens of Syllaemus and Leptichthys. Further, the scales of the 

 latter two genera have a large and diffuse nuclear field mainly devoid 

 of any ornamentation and, depending on body position, with the 

 apical quadrant covered proximally, either with complete and entirely 

 vertical cLrculi or with a median hyaline zone. Such comparisons 

 prompt, in at least tentative solution of the problem, allocation of the 

 herycinus species to the oldest generic name unquestionably asso- 

 ciated with this group, Apsopelix. 



To be demonstrated shortly in another connection, the compact 

 assemblage of fishes under consideration is of elopine stock. The 

 members, thus, display interesting specializations of the most 

 primitive teleostean stage of morphologic organization. They 

 comprise a distinctive component of upper Cretaceous marine faunas 

 and would seem best interpreted as a line of pelagic plankton feeders. 



Sincere thanks are extended to Drs. Bobb Schaeffer, American 

 Museum of Natural History, and Robert W. Wilson, Kansas Uni- 

 versity Museum of Paleontology, for courtesies in making pertinent 

 materials available. The photograph was made by Jack Scott of 

 the Department of Geology, U. S. National Museum. 



