TELEOSTEI 



345 



centra ; the intermuscular bones ; the homocercal tail with hypural 

 bones ; the further reduction of the radials of the paired fins ; the 

 spine-like postclavicle ; the supraoccipital (p. 326); the unpaired 

 vomer ; the simplification of the lower jaw, which loses the supra- 

 angular and splenial, preserving only the dentary angular and 

 articular ; the absence of special cheek-plates, and loss of the 

 median gular ; the median urohyal ; the reduction of the conus 

 and its valves, and compensating development of the truncus 

 arteriosus ; the loss of the optic chiasma and of the spiral valve ; 

 the special vas deferens, and ovarian sac. 



Many of these characters are not possessed by the lower forms, 



cor 



n, 



pm 



sop 



TX>P wp 



FIG. 326. 



Eight-side view of the skull of Slops saurus, L. (From Ridewood, Proc. Zool. .S'oe.) cor, cir- 

 cumorbitals ; d, dentary; fear, dermarticular ; /, frontal; hm, hyomandibular ; iop, inter- 

 opercular ; rnx, maxilla ; n, nasal ; ope, opercular ; pm, preniaxilla ; pop, preopercular ; pt, post- 

 temporal ; q, quadrate ; sm, supramaxillaries ; soji, subopercular ; .-.?, snpratemporals. 



either because the new structures have not yet become developed 

 or because certain ancestral characters are not yet lost. The 

 Teleostei are the "most recent of all the Actinopterygian Orders; 

 they are not known to occur below the Jurassic, and thus offer to 

 the systematist at once the best opportunity for tracing out 

 phylogeny, and a most confusing number of intermediate forms. 

 For the most recent advances in the classification we are chiefly 

 indebted to Gill [165], Giinther [151-92], Sagemehl [379], Jordan 

 [250-51], A. S. Woodward [505], and Boulenger [40-42]. 



In the skull we notice a general tendency, already referred to 

 (p. 222), for the dermal bones to sink deep below the surface, 

 leaving lateral-line ossicles in the skin (Allis [16, 19]), Parker [319], 

 .Schleip [387], Gaupp [151rt]. The prefrontal (ectethmoid, 



