xxj 



SKULL 



well- developed hyomandibular and sym- 

 plectic, and the middle portions of the 

 ceratohyals and ceratobranchials are also 

 invested by bone. 



Dermal bones are well developed both 

 on the upper surface and sides of the 

 head. The rostrum and occipital region 

 are covered by numerous bones, and the 

 typical series of paired bones on the roof 

 of the skull may become separated by an 

 intercalated secondary series of median 

 membrane bones. In the upper lip a 

 maxilla, but no premaxilla, is developed, 

 in the lower lip a dentary bone appears. 

 On the roof of the mouth there is in 

 front a single bone representing the 

 vomer, and behind it an immense para- 

 sphenoid. The region of the pectoral 

 girdle is covered by membrane bones 

 having in general the same arrangement 

 as -in Teleostei, but between the two 

 cleithra there are two clavicles which 

 meet in the mid" ventral line. These 

 bones, absent in Teleostei, are present 

 in most land animals, and must have 

 been inherited from the common an- 

 cestor from which fish and land animals 

 are derived. 



Living Chondrostei fall into two 

 families, viz. the Acipeuseridae, including 

 Acipenser and Scapkirhynckus, which 

 possess five longitudinal rows of great 

 bone scutes on the body, which appear 

 to merge insensibly into the dermal 

 bones of the head, and the Polyodon- 

 tidae, in which the body is naked except 

 for vestigial scales embedded in the skin, 

 and there is no series of median dermal 

 bones on the roof of the head. The 

 dorsal lobe of the tail fin retains, how- 

 ever, its series of fulcra (Fig. 244). 



Caviare is prepared from the ovaries 



FIG. 244. Acipenser sturio, 

 the Sturgeon. From Day. 



