266 THE PLAGIOSTOMIA. 



it is found that the distance from its base to that of the pectoral is about 4, the 

 distance from the base of the pectoral to that of the ventral is also 4, the distance 

 from the origin of the ventrals to the end of the caudal is 152, that from the bases 

 of the ventrals to the end of the caudal 14, the length of the caudal 6$, and the 

 total length 38 inches. The teeth on the rostrum of this specimen stand out a 

 little more than one fourth of an inch; individuals differ in lengths of these teeth. 

 Latham's example had teeth a fourth of an inch in length. The figure given by 

 Day fairly represents the species as represented here, in specimens secured by 

 Prof. L. Agassiz, on the Thayer Expedition, in the Amazon River and tributaries. 

 On a specimen of about fifteen feet in length, the rostrum has a length of about 

 four feet; at the base it is eight inches in width, between the outermost teeth 

 four inches, and the longest teeth have lengths of more than two inches. 



Rhinobatidae. 



Body, head, and tail depressed. Disk broad posteriorly, tapering forward; 

 pectorals continued opposite the gill openings but not in the snout. Tail strong, 

 wide at the base, with two dorsals, a moderate to small caudal fin, and a dermal 

 fold at each side. Nostrils oblique; each valve in three sections, an outer 

 section and an inner separated by an elongate lobe. Teeth small, numerous, 

 in pavement. Orbit with a fold below the eye and a projecting shield above the 

 pupil. Spiracles large, close to the eye. 



The genera of this family should be grouped in two subfamilies, one of them, 

 the Rhynchobatinae, to include species on which the dorsal is placed above the 

 ventrals, the caudal axis is raised and there is a well-developed subcaudal lobe, 

 the other, the Rhinobatinae, to contain those on which the dorsal is at a distance 

 behind the ventrals and the subcaudal is small and without a lobe. 



A number of fossil species undoubtedly belonging to this family are known 

 from the Lower Kimmeridgian, Upper Cretaceous. Upper Eocene, and Later 

 Tertiary. 



Ventrals at a distance from the pectorals; subcaudal lobed 



dorsal opposite the ventrals Rhynchobatinae 



rostrum short; snout, broad, rounded . Rhamphobatis (page 267) 

 rostrum long; snout pointed . . Rhynchobatus (page 268) 

 Ventrals close to the pectorals; subcaudal small, not lobed 



dorsal behind the ventrals Rhinobatinae 



rostrum long; snout pointed . . . Rhinobaius (page 269) 



