ACTINOPTERYGII. 115 



traverse this deepened series, but turns down the second or third scale 

 and extends along the series of normal scales immediately below. The 

 fins are small, all fringed with minute fulcra; the dorsal and anal fins 

 are slightly extended and directly opposed ; the caudal is deeply forked. 

 Pleuropholis is essentially Upper Jurassic, having been found only in 

 the Lithographic Stone of Bavaria and Ain, France, in the Portlandian 

 of France, and in the English Purbeck Beds. P. laiwaxima is the best- 

 known species from Bavaria, ajid P. attenuata is the typical species from 

 the Purbeck of Sutton Mandeville. P. crassicauda and P. longieauda 

 occur in the Purbeckian of Swanage, while P. serrata belongs to a similar 

 horizon at Hartwell, near Aylesbury. 



The Leptolepidae exhibit an advance upon the Pholido- 

 phoridae only in four particulars. The vertebral centra are 

 always well ossified in the adult, a few intermuscular bones are 

 developed, fin-fulcra are wanting, and the scales are never 

 united by a peg-and-socket articulation though apparently 

 always coated with a thin film of ganoine. These fishes range 

 from the Upper Lias at least to the Lower Cretaceous, and 

 become modified only by the strengthening of the vertebral 

 centra and the multiplication of the intermuscular bones. 



Fio. 80. 



Leptolepis dubius ; restoration, scales omitted, about one-third nat. size. 

 U. Jurassic (Lithographic Stone); Bavaria. (From Brit. Mus. Catal.) 



Leptolepis (fig. 80). The head is shaped like that of Pholidophorus, 

 and the gular plate is similarly wanting. The radiating branches of the 

 suborbital sensory canal have been observed on the large preorbital cheek- 

 plate. The mandible is definitely proved to consist of only two ossified 

 elements, a dentary and an articulo-angular, and the former suddenly 

 rises into a thickened ascending process at a short distance from the 

 symphysis (suggestive of the same bone in the carps). The vertebral 

 centra are constricted cylinders in close series, but are always pierced 

 by the continuous notochord ; they are simple in the Liassic species, but 



82 



