xiii PHYLUM CHORDATA 213 



produced into enamelled spines and articulating with one another 

 by suture. Scutes are also found in many Siluroids (Fig. 827) 

 and in Lophobranchii (Fig. 832) and some Plectognathi (Fig. 831), 

 while in the Plectognathi the exoskeleton takes the form, as in the 

 File-fishes, of minute spines like the shagreen of Sharks, or, as in 

 many Globe-fishes, of long, outstanding, bony spines. Lastly, in 

 Polypterus and Lepidosteus are found rhomboid or ganoid scales 

 (Fig. 835, B), in the form of thick, close-set, rhomboidal plates 

 formed of bone, covered externally by a layer of enamel or ganoin, 

 and joined together by pegs and sockets. In many Ganoids the 

 anterior fin-rays of both median and paired fins bear a row of 

 spine-like scales called fulcra (Fig. 825, fl.). 



Endoskeleton. In the Sturgeon the vertebral column (Fig. 837 

 WS.) consists of a persistent notochord with cartilaginous arches 



FIG. 830. Anterior end of vertebral column of Polypterus. PS. parasphenoid ; R, I V, dorsal 

 ribs ; ll'K, centra ; t, ventral ribs. (From Wiedersheim's Comparative Anatomy.) 



and is fused anteriorly with the cranium. In the remaining 

 orders bony vertebrae are present ; the centra are biconcave, except 

 in some Eels, in which the anterior face is flat or even convex, 

 and in Lepidosteus, in which the anterior face is distinctly convex. 

 Vertebrae of this form, i.e. having the centrum convex in front and 

 concave behind, are called opisthoccelous. Ribs are usually present: 

 in Polypterus each vertebra has two pairs, a dorsal pair (Fig. 836, 

 E, I V) of considerable length, running between the dorsal and 

 ventral muscles, and a short ventral pair (f) between the muscles 

 and the peritoneum : the former answer to the ribs of Elasmo- 

 branchs, the latter to the ribs of the remaining Teleostomi, which 



