934 



CLASS PISCES. 



all those of the female are obtuse. Some of these Skates measure 

 upwards of seven feet across the disk. In a fossil state this genus 

 occurs in the Chalk of the Lebanon, and the Upper Eocene of 

 Hampshire ; and also in the Suffolk Crags and Italian Pliocene, 

 where it is represented by the living R. clavata (figs. 856, 857). 

 The extinct Dynatobatis, from the Tertiary of South America, is dis- 

 tinguished by the enormously expanded bases and the small spines 



Fig. 856. — Front view of the jaws of the male Thornback Skate {Rata ciavata). Reduced. 



of the dermal tubercles with which the body is studded. Acantho- 

 batis, from the Middle Miocene of France and Wiirtemberg, has tall 

 dermal tubercles, with small bases, which often fuse together into 



Fig. 857. — Front view of the jaws of the female Thornback Skate {Raia clavata). Reduced. 



groups ; while Oncobatis of the Pliocene of Idaho, which has these 

 tubercles of a pentagonal form, may be identical with the type 

 genus. 



Family Torpedinid^. — The Electric Rays have the disk broad 

 and smooth ; the rayed portion of the pectoral fins not continued 

 beyond the base of the snout, and the median fins well developed. 

 The peculiar electric organ is placed between the head and the 

 pectoral fins. Extinct species of the existing genus Torpedv occur 

 in the Middle Eocene of Monte Bolca, near Verona. 



Family Psammodontid/e. — This extinct family is known only by 

 portions of the dentition, so that its definition is at present impos- 

 sible. It appears, however, from the parallelism of the mandibular 

 rami that the body must have been depressed like that of the Rays. 

 The teeth (fig. 852, 5) are of a flattened quadrangular form, with 

 the root nearly as large as the crown, and were arranged in one or 

 more longitudinal rows, which were arched antero-posteriorly with 



