346 AN INTRODUCTION TO THE STUDY OF FOSSILS 



Order b, Chondrostei. — Caudal fin heterocercal. This order 

 includes Cheirolepis from the Devonian of North America and 



f 



Fig. 147. — A well-preserved crossopter>-gian fish, Eusthenopteron foordi Whiteaves, 

 nearly three feet long from the Upper Devonian of Scaumenac Bay, Quebec. The 

 paired fins of both sides are shown. The vertebra? had ossified only in the anterior 

 portion of the notochordal sheath, though neural and haemal spines continue well- 

 developed to the caudal fin. The internal cartilaginous supports of the fins are 

 well preserved. A., anal fin; B.C., basal fin cartilages; C, caudal fin; this is 

 diphycercal ; CI., clavicle ; D^, D2, anterior and posterior dorsal fins ; D.R., dermal 

 fin rays; Fr., frontal; G., gular plate; H ., haemal spines; I.CL, inf raclavicle ; 

 L.P., left pectoral fin ; L. V ., left ventral or pelvic fin ; Mo., mouth ; Mnd., mandible ; 

 M.S., median supratemporal ; Mx., maxilla; N ., position of notochord; N.Sp., 

 neural spines; Op., operculum; Pa., parietal ; P. il/.r., premaxilla; P. Op., pre- 

 operculum; PT ., post-temporal; Pt.F., post-frontal; R.C., radial fin cartilages; 

 R.P., right pectoral fin; R.V ., right ventral or pelvic fin; S.Cl., supracl avicle ; 

 SO., suborbitals; S.Op., suboperculum ; ST., lateral supertemporal ; Su.O., 

 supraorbitals ; V., vertebral centra. (After Hussakof.) 



Europe, Catopterus from the Triassic of North America, and 

 the living sturgeons from seas of the Northern Hemisphere; 

 the latter enter the various rivers of North America, Europe 

 and Asia. 



