76 



FISHES. 



the base of the long projecting snout. The suspensormm is 

 movably attached to the side of the skull, and consists of 

 two pieces, a hyomandibular and a symplectic, which now 

 appears for the first time as a separate piece, and to w^hich 

 the hyoid is attached. The palato-maxillary apparatus is 

 more complex than in the Sharks and Dipnoi; a palato- 

 pterygoid consists of two mesially-connected rami in Polyodon, 

 and of a complex cartilaginous disk in Acipenser, being arti- 

 culated in both to the Meckelian cartilage. In addition, the 

 Sturgeons possess one or two pairs of osseous rods, which, in 

 Folyodon at least, represent the maxillary, and therefore must 

 be the representatives of the labial cartilages of the Sharks. 

 The Meckelian cartilage is more or less covered by tegu- 

 mentary bones. 



In the gill-cover, besides the operculum, a sub- and inter- 

 operculum may be distinguished in 

 Acipenser. 



The hyoid consists of three pieces, 

 of which the posterior bears a broad 

 branchiostegal in Polyodon. 



In the scapulary arch the primordial 

 cartilaginous elements scarcely differ 

 from those of the Dipnoi. The mem- 

 brane-bones are much expanded, and 

 offer a continuous series suspended from 

 the skull. Their division in the median 

 ventral line is complete. 



The pectoral is supported by a car- 

 tilaginous framework (Fig. 38) similar 

 to that of Ccratodm, but much more 

 shortened and reduced in its periphery, 

 the branches being absent altogether on one side of the axis. 

 This modification of the fin is analogous to the heterocercal 

 condition of the end of the spinous column. To the inner 



Fis 



38. — Fore-limb of 

 Acipenser. 



