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 lacunae, and therefore 



approach in structure 



A Q 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- 

 ARCH A. 



This group contains 

 five genera, of which 

 Pterichthys (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 



Fia. 931. Pterichthys testudinarius. A, dorsal B, (^. /.) of a very re- 

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

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



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 Csecilians, and the extinct Stegocephala 

 or Labyrinthodonts. 



