MUSEUM OF COMPARATIVE ZOOLOGY. 95 



front of the eye, the short cranials have a very sharp and prominent bend; on 

 the rostrum they are close together and nearly parallel. From the branch 

 sent back of the spiracles the orbitals incline a little outward, and proceed 

 thus until more than half-way to the edge, when they turn inward; close to 

 the rostrals they again take a longitudinal course for a short distance, and find 

 their way down, in front of the pleurals, after sending out upon the fin ten or 

 a dozen long tubules. 



On making their appearance on the lower surface the pleurals pass directly 

 forward; nearing the margin they turn and follow it around, gradually reced- 

 ing from it, to a point opposite the mouth, where they turn toward the abdomi- 

 nal chamber. Back of the shoulder girdle they turn slightly outward; and 

 when opposite the pelvis they turn toward it abruptly, meeting the extension 

 from the jugular at the edge of the abdomen. On the transverse posterior por- 

 tion, near the pelvis, there are a few tubules of medium length ; on the 

 portions anterior to the gills there is a multitude of tubules that reach to 

 the edge of the disk. The suborbitals emerge a little in front of the pleurals, 

 which they cross, to run obliquely back until not far in front of the gills, 

 where they take an inward and forward course to meet the angulars a little 

 back of the mouth. As they pass backward they send off nine or ten branches 

 which by repeated forkings and fusions form networks, the outer limits of 

 which are the pleurals, and in which the inner areas are large and elongate 

 polygons and the outer small and short ones. The orbito-nasals are of moder- 

 ate length, connecting, as in the majority of the Sharks, with the angular and 

 suborbital posteriorly, and with the subrostral and nasal anteriorly. Each sub- 

 rostral makes a very prominent bend in front of the nostril; it does not return 

 far enough to reach the nasal valve; and it ends at the side of the prenasal, 

 near the skull, in a series of four or five swellings or follicles. No great 

 amount of curvature appears in the. nasals. The median is very short. The 

 prenasals are close together and nearly parallel; they have several irregularly 

 placed rings or bunches along their sides. Not far from each angle of the 

 mouth there is a short disconnected oral. 



The sketches were made entirely from the left side of the specimen. 



This genus is well distinguished from its allies, the Potamotrygons, on the 

 one hand, and the Thalassotrygons, on the other, by the disposition of the tubes 

 on the shoulders, and the orbito-pleural plexus beneath the pectorals. 



It is quite possible that the appearance of the follicles on certain of the 

 canals of the ventral surface, attended by deterioration and disappearance of 

 the tubes themselves, in this genus and Urolophus, and in species of other 

 genera, points toward a change made from habits similar to those of the typi- 

 cal Thalassotrygons, in which the lower canals possessed great utility, to others 

 leading the individual to remain habitually on the bottom, where the lower 

 vessels may be comparatively useless, which if persisted in lead to disuse 

 and ultimate obsolescence of the tubes, as in the Torpedinidse. It is not far to 

 the conclusion that, through their ancestors, Torpedoes, as well as Potamotry- 

 gons, were more closely related to the Thalassotrygons. 



