256 IOWA GEOLOGICAL SURVEY 



BelonorhyncMdae. On the other hand, there are certain fea- 

 tures which indicate an approach to Fishes of an altogether 

 more modern type. Finally, the Chondrostei represent a stage 

 in a career of degeneration, the climax of which is reached by 

 the modern Polyodontidae and Acipensideridae. " 



Family palaeoiniscidae. 



Primitive Chondrostei with fusiform bodies, short dorsal and 

 anal fins, and usually with a complete investment of articulat- 

 ing rhombic, rarely cycloid, ganoid scales. Fulcra generally 

 present at the bases of the median fins, and especially along 

 the dorsal border of the upper caudal lobe. Ribs are not known 

 to be present. Skull invested by a very complete series of paired 

 dermal bones, which in number and disposition conform to the 

 normal Teleostome type. The secondary upper jaw includes 

 both prernaxilla? and large maxillae ; and, as a rule, both the den- 

 tary and splenial bones of the lower jaw are dentigerous. Ex- 

 cept for the absence of an interoperculum, the opercular series 

 of bones is complete, including numerous branchiostegal rays. 

 There is a single small median jugular plate. 



The earliest representative of this family, and of primitive 

 sturgeons generally, is the genus Cheirolepis, which is remark- 

 able for the very small size of its scales. Long associated with 

 Acanthodian fishes, even by experienced observers, its Palae- 

 oniscid nature was first completely demonstrated by Dr. Earn- 

 say H. Traquair* in 1875, who was also the first to point out 

 the presence of a median jugular plate in typical genera. The 

 type and best known species of Cheirolepis, C. trailli Agassiz, 

 occurs in the Old Red Sandstone of northern Scotland ; and an- 

 other well preserved form, described by TVhiteaves as C. cana- 

 densis, is known from the Upper Devonian of Scaumenac Bay, 

 in the Province of Quebec, 



Aside from examples of the Canadian form just mentioned, 

 no completely preserved representatives of this family are 

 known from the Devonian rocks of North America, although 



* Traquair, It. H., On the structure and systematic position of the genus 

 Cheirolepis. Ann. Mag. Nat. Hist. (1875). ser. 4, vol. 15. pp. 237-L>49. A similar 

 suggestion had been made tentatively two vears earlier bv Dr. Karl Martin (Zeit- 

 schr. deutsch. Geol. Ges. 1873, 25, p. 699). 



