MUSEUM OF COMPARATIVE ZOOLOGY. 69 



The Torpedinidae have lost the canals of the lower surface. Rem- 

 nants of the missing vessels are found in the follicles of Savi, present 

 ou some. The pleurals and the orbitals unite directly, without the 

 intervention of tubules. In Narcine the entire extent of the system is 

 much less than in Torpedo, owing to the reduction in number and length 

 of the tubules. Yet in these respects there is not a little variation in 

 the species of Torpedo, as is seen by comparison of T.' calif ornica and 

 T. inarmorata. 



In the Trygonidae, as, further along, in the Myliobatidse and their 

 allies, we see a disposition to enlarge the system by means of curves, 

 tubules, and dichotomous branchings much beyond what has been no- 

 ticed in the Skates. Through the increase in length of the main tubes 

 the courses of orbitals and suborbitals have come to be crossed by the 

 pleurals on both upper and lower surfaces. The presence of a small 

 enclosure, or more than one, on each shoulder, formed by scapular 

 branches, pre-pleural or post-pleural, or both, is somewhat general in this 

 section of the Batoids. 



Potamotrygones as well as Thalassotrygones have the tubules of the 

 pleurals on the lower surface massed anteriorly, comparatively few 

 appearing under the posterior half of the disk. An obsolescent condi- 

 tion of the subrostrals obtains in Disceus and Potamotrygon ; where 

 parallel with the prenasals these tubes are merely lines of follicles, 

 without apparent connection by their cavities, marking the paths of the 

 canals. On the lower portion of the pleural of Potamotrygon there are 

 rather few tubules ; the sections of the oral are elongate and sinuous ; 

 the nasal meets the angular, and apparently there is a short sternal 

 canal. On Disceus the tubules are very numerous on the lower pleu- 

 rals, the parts of the oral are short and separated by some distance, 

 the nasal and the subrostral meet, and there is an orbito-pleural plexus 

 containing a large number of small areas. Differences similar in char- 

 acter, but less pronounced, exist on the upper surfaces of these genera. 



Urolophus has no orbito-pleural plexus on the lower surface, its pleu- 

 ral tubules are not massed in front, and the suborbital is not provided 

 with a long loop pointing forward as in Potamotrygon. It has a short 

 sternal tube. 



Tseniura resembles Urolophus more than it does the Potamotrygons. 

 Like the former it has the pleural tubules distributed along the tube, 

 and it has neither orbito-pleural plexus nor suborbital loop. It differs 

 from Urolophus in the multitude of its branchlets on the upper aspect, 

 in its pleural areas, and in the union of subrostral and nasal. 



