936 



CLASS PISCES. 



North America ; and it has also been described from the Tertiary of 

 New Zealand. A very large number of specific names have been 

 applied to the fossil forms, but Mr S. Woodward has shown that the 

 number of valid species may be greatly reduced, since many of the 

 characters on which they were founded are solely due to differences in 

 the age of the specimens. In Rhinoptera (Zygobates), while the head 

 is still free, there are two cephalic fins, and the teeth are arranged in 

 five or more rows, of which the middle series and the adjacent pair 

 are broad, while the one or two pairs on the borders form regular 

 hexagons. This genus is known from the London Clay, the Eocene 

 of South Carolina, the Swiss Miocene, and the English Crag, and is 

 now represented by seven species from tropical and sub-tropical seas. 

 The existing genus Aetobatis is distinguished from the preceding 

 by the teeth being arranged in only a single longitudinal row, which 

 is often bent, and corresponds to the median row of Myliobatis. 

 One species is found from the London Clay to the higher Eocene 

 of Barton, and the genus is also represented in the Swiss Miocene. 

 The existing genus Ceratoptera has no upper teeth. 



In this family may be provisionally included the Cretaceous genus 

 Ptychodus, which was long considered to be a Cestraciont Shark, 

 but has been shown by Mr Smith- Wood- 

 ward to be a Ray allied to Myliobatis. 

 The connection between Ptychodus and 

 the existing Myliobatidce is shown by the 

 genus Apocopodon, from the Upper Cre- 

 taceous of Brazil, which has teeth of an 

 intermediate type ; while some of the 

 Eocene species of Myliobatis have teeth 

 of nearly the same form as those of 

 Ptychodus. The teeth of this genus (fig. 

 859) have quadrangular crowns, with the 

 enamel of the central region thrown 

 into a number of transverse folds, while 

 the root is smaller and lower than the 

 crown. The two sides of each jaw are 

 parallel to one another, and the teeth are 

 arranged in several parallel rows running 

 from back to front, as is shown in the 

 accompanying diagram (fig. 860). It will be 

 seen, moreover, that each jaw has a single 

 median series, composed of very small teeth in the upper, and 

 of very large ones in the lower jaw ; while on either side of this 

 median row there is a series of teeth somewhat less large than 

 the median row of the lower jaw. The five external rows gradually 

 decrease in size towards the outer side of the jaws. A comparison 



Fig. 859. — Oral and lateral 

 views of the crown of a tooth of 

 Ptychodus fiolygyrus, from the 

 Upper Greensand of Regens- 

 burg. (After Zittel.) 



