ORDER ELASMOBRANCHEI. 



935 



some alternation of the teeth of adjacent rows. The genus Copodus 

 has symmetrical teeth, with the postero-lateral angles of the root, 

 and sometimes also of the crown, produced backwards ; the teeth 

 being narrowest in front, with the anterior margin usually either 

 straight or convex. The crown surface when unworn is rugose, and 

 marked by a more or less transverse line, which sometimes permits 

 the two portions to be separated. This genus occurs in the Car- 

 boniferous of Europe. In the typical Psammodus the teeth (fig. 

 852, 5) are quadrangular, generally more or less oblong, but occa- 

 sionally nearly square, with the root much thicker than the crown, 

 from which it is easily separable ; the surface of the crown being 

 generally marked by transverse wrinkles. It is probable that the 

 teeth were arranged in the jaws in four longitudinal rows. Psam- 

 modus is represented by a considerable number of species from the 

 Carboniferous of both Europe and North America. Lastly, Archcz- 

 obatis, from the Carboniferous of Indiana, is an allied form, with 

 very large pavement-like teeth arranged in several rows, of which 

 the under surfaces are somewhat excavated to fit the curvature of 

 the jaws. 



Family Myliobatid^e. — With the Eagle-rays we come to an exist- 

 ing family well represented in a fossil state as far down as the Lower 

 Eocene ; vertebrse from Cretaceous and Jurassic beds having been 

 also referred to this family. The disk is very large, owing to the 

 great development of the pectoral fins, which stop short at the sides 

 of the head, but reappear at the extremity of 

 the snout in the form of a small single or 

 paired cephalic fin. The tail is extremely 

 slender, and resembles a whip-lash; and the 

 dentition, when present, forms a complete 

 pavement. The type genus Myliobatis has 

 the head free from the disk, and a single 

 cephalic fin. The teeth are large, flat, and 

 hexagonal, and are arranged in seven longitu- 

 dinal rows ; the middle row in the adult being 

 extremely broad, while the lateral rows are 

 lozenge-shaped (fig. 858). In the young the 

 middle row of teeth is not larger than the 

 lateral ones, and there is a gradual increase in 

 the relative breadth of this row as the fish in- 

 creases in age. The upper dental plate is ex- 

 tremely convex from before backwards, but the 

 lower one is quite flat. In addition to the 

 doubtful vertebrae mentioned above, this genus is known continu- 

 ously from the Lower Eocene upwards, and has a wide distribution in 

 space. Thus, in the Eocene, it is recorded from Europe, India, and 



vol. 11. d 



Fig. 858. — Part of the 

 lower dental plate of My- 

 liobatis striata, from the 

 Middle Eocene of Brackle- 

 sham, Sussex. Reduced. 

 The second lateral rows-of 

 teeth are imperfect, and the 

 third wanting. 



