256 



ZOOLOGY 



SECT. 



near the middle of the dorsal surface. Behind the shield, towards the ventral 

 surface, is a plate which perhaps supported the operculum (C, op.}, but may 

 represent the pectoral fin. The scutes contain some lacume, and therefore 



approach in structure 



A B to bone. The posterior 



portion of the body is 

 covered by deep, nar- 

 row scales ; there is a 

 single dorsal fin and a 

 heterocercal caudal. 



ORDER, 3. ANTI- 



ARCHA. 



This group contains 

 five genera, of which 

 PterichtJiys (Fig. 931) 

 may be taken as an 

 example. It presents 

 a broad and high an- 

 terior region, covered 

 by articulated plates 

 which have the struc- 

 ture of bone and are 

 covered by a layer of 

 enamel, and a caudal 

 region covered by 

 rounded or hexagonal 

 scales. The orbits are 

 placed close together 

 on the top of the head, 

 and between them is a 

 plate pitted on its 

 inner surface, ap- 

 parently for the pineal 

 body. There is a pair 

 of large pectoral fins 

 (pet. /.) of a very re- 

 markable character, 

 covered by strong 

 scutes and divided 



into two parts by a joint towards the middle ; a single dorsal fin (d. /.) with 

 fulcra, but apparently no fin-rays ; and a heterocercal tail-fin (c. f.). 



FIG. 931. Pterichthys testudinarius. A, dorsal, B, 

 ventral, C, lateral aspect, c. f. caudal fin ; d.f. dorsal fin ; 

 pct.f. pectoral fin. (From the Brit. Mus. Cat. of Fossil Fishes.) 



CLASS IV. AMPHIBIA. 



The Amphibia are distinguished from Fishes by the possession 

 of pentadactyle limbs instead of paired fins, and by the absence of 

 fin-rays in the median fins. They nearly all breathe by gills in 

 the larval condition, and many of them retain those organs through- 

 out life ; lungs are, however, usually present in the adult. The 

 class includes the Frogs, Toads, Newts and Salamanders, as well 

 as the peculiar snake-like Caecilians, and the extinct Stegocephala 

 or Labyrinthodonts. 



