44 RAYS. 



ral form is that of a broad base or plate from which a short spine is 

 produced ; this is most conspicuous in the central rows of teeth where 

 the spine is developed from the inner angle, but it gradually dimi- 

 nishes as the teeth approach the sides of the jaws. The vertical series 

 of teeth describe nearly a complete circle at the middle part of the 

 jaws, and include about twenty teeth each. 



The teeth of the Torpedo are somewhat stronger, their base is 

 more extended transversely, and the pointed cusp is more produced 

 than in the common rays. 



In the thornback {Raia clavata), the teeth present the form of 

 small transversely oval, obtuse masses ; and the vertical series are so 

 closely arranged, that being alternate, they are less conspicuous than 

 the oblique rows which result from this approximation. Mr. Yarrell 

 describes the following sexual character in the teeth of the Thornback 

 ray. *' While both sexes are young, the teeth in both are alike broad 

 and flat ; but as the male acquires age and sexual power, the teeth 

 that are nearest the centre begin to alter in form, and become pointed 

 by an elongation of the internal angle ; all the points being directed 

 backwards or towards the throat. "(1) 



The quincuncial arrangement of the teeth prevails in the different 

 species of Trygon, or fire-flare, in which the teeth are of very 

 small size. Their crowns consist of a triangular plate, with the 

 internal angle or point most produced ; this plate is supported on a 

 hollow pivot. 



In the genuS Rhina each tooth is supported on a short fang or 

 pivot, which tapers as it recedes from the crown ; there is a groove 

 along the posterior part of this pivot, and a perforation on each side ; 

 the crown is lozenge-shaped, convex above, and sculptured with a 

 series of transverse and shghtly undulating and punctate ridges ; 

 presenting a pattern which somewhat resembles that of the grinding 

 surface ofthe comparatively gigantic tooth of the extinct Ptychodus. (2) 

 The modification of the dentigerous surface of the jaws, and the beau- 

 tiful quincuncial arrangement of these teeth are exhibited in PL 23, figs. 

 ], and 2. The middle part of the upper jaw forms a bold prominent 



(1) British Fishes, ii, p. 416. 



(2) Compare the magnified view of a denticle of RJiina in PI. 23, fig. 3, with the tooth of 

 Ptychodus latissimus, PI. 17, figs. 1 and 2. 



