3»6 



A CIPENSEROIDEI 



lamellae are attached to a septum for about half their length 

 (p. 95). The brain and other internal organs are of generalised 

 structure (p. 305). 



The scales possibly preserve some true ganoine in some of the 

 early genera ; but they never show any trace of the cosmine or of the 

 network of vascular canals seen in Palaeoniscids. In Acijpenser 

 they are formed merely of concentric layers of bone. 



The chief degenerate characters of the more recent forms are 

 shown by the loss of the ganoine on the dermal skeleton, the loss of 



j^ rv. . km- pt- 



Ion folimn, Lac. (After Traquair.) A, left-side view of skull, jaws, and pectoral girdle; 

 B, inner view ofrightjaws. c, coracoid : -■/(, ceratohyal ; i ■/. da\ icle : clt, cleithrum ; d, dentary ; 

 he, postclavicle : ftm,, hyomandibular : l.m, levator muscle ; m, Meckel's cartilage : nue, maxilla ; 

 «, olfactory capsule; o, optic, capsule; op, opercular; pa, palatine; ps.t, post-temporal; pt, 

 pterotic : so, subopercular ; spl, splenial ; •■••</. syinplectic. 



scales on the trunk, of branchiostegal rays, of the median gular, and 

 the reduction of the opercular bones ; the loss of fulcra, except on 

 the tail ; the irregularity and subdivision or loss of the dermal 

 cranial bones ; the loss of the premaxillae and reduction of the 

 teeth ; the appearance of a median series of bones on the skull ; the 

 relative lack of ossification in the endoskeleton. There seems, on 

 the other hand, to be a compensating recrudescence of cartilage. 



Family Chondrosteidae. The trunk appears to be scaleless; rhombic 

 ganoid scales are found on the tail only. The paired frontals, parietals, 

 postfrontals, and pterotics (squamosals) are still normally developed, and 

 not separated by a median series (Fig. 295). 



