ORDER ELASMOBRANCHEI. 



929 



ing ; while specialisation shows itself in the depression of the body 

 and the enlargement of the pectoral fins — the spiracles being always 

 retained. This section includes the Spiny Dog-fishes, Saw-fishes, 

 Eagle-rays, and Rays. 



Family Spinacid^e. — In this family, which includes the exist- 

 ing Spiny Dog-fishes (fig. 849), we have generalised forms, with the 

 body more or less rounded, and but slightly depressed. The teeth 

 are pointed ; the pectoral fins are devoid of a notch at their root, 

 and are not expanded anteriorly ; while the gills are small and 

 lateral, and the spiracles large. One fossil species referred to the 

 existing Mediterranean genus Centrina has been recorded from the 

 Pliocene of Italy ; but this determination is not absolutely certain. 

 Acanthias (fig. 849), of which two species are found at the present 

 day in the temperate seas of both hemispheres, occurs in the Chalk 

 of the Lebanon, and also in the Miocene of Wiirtemberg. Another 

 fossil form from the Lebanon has been referred to the existing genus 

 Centrophorus, but it may perhaps belong to Acanthias. The exist- 

 ing genus Spinax has been recorded from the Italian Pliocene. 

 With Scymnus we come to another existing genus, differing from all 

 those that precede by the absence of fin-spines. It occurs fossil in 

 the Pliocene of Italy, which has also yielded remains referred to the 

 allied Echinorhinus. 



Family PETALODONTiDiE. — The Petalodonts form a family ex- 

 clusively Carboniferous, presenting the following characters. The 

 body was somewhat depressed, while the pectoral fins were large, 

 and produced forwards in the direction of the head after the manner 

 of the Rays. The teeth (figs. 

 851, 852) formed a close pave- 

 ment in the mouth, and are com- 

 pressed from before backwards, 

 with the crown more or less bent 

 backwards, and either blunt and 

 obtuse, or with a cutting-edge, the 

 root being often large. In the 

 genus Janassa (Climaxodus or 

 Strigilina), which is common to 



Fig. 851. — Posterior aspect of some of the 

 central teeth of Janassa linguceformis ; from 

 the English Carboniferous. 



Europe and North America, the 

 teeth (fig. 851) are so thickened 

 and reflected, that the complete 



series forms an almost entirely triturating surface. These teeth are 

 arranged in three chief rows, as in the figure, which gradually 

 diminish in size anteriorly, and are flanked by one or more smaller 

 rows of less thickened teeth. The body is covered with fine sha- 

 green. The North American Fissodus has the margin of the crowns 

 of the teeth cleft into two or three points ; while in Petalorhynchus, 



