Garman.] 204 [May 2, 



two. Differentiation of the anterior portion of the ventral and its 

 use as a sort of limb direct attention to the articulations at the end 

 of the shaft. 



Two quite different patterns are to be seen in the species of the 

 Trygonidae, one of which appears only among those frequenting the 

 rivers of South America, and another common to all the marine spe- 

 cies. The former may be designated as the subfamily Potamotry- 

 gones, the latter as that of the Thalassotrygones (characterized below). 

 Differences of general structure and habits accompanying those of 

 special interest in this communication give ample grounds for the 

 separation. 



One of the most peculiar pelves in the order is that of the river 

 Trygons. Compared with the size of the body, the shaft is short. 

 It is very strong and nearly straight. From the middle of its length 

 a strong tapering cartilage extends more than half the distance to 

 the shoulder girdle, — a peculiarity not shared by the marine species 

 of the family. Schomburgk, to whom we are indebted for what we 

 know of the habits of the South American trygons, gives a hint as to 

 the probable utility of this subabdominal cartilage. Speaking of 

 species of one of the two genera making up the' division, he says 

 " they dig round holes five or six inches in depth, in which they lie, 

 sometimes partly covered with sand, in order easier to entrap their 

 prey." 



Since the soft flexible borders of the disk are wholly unfit for dig- 

 ging purposes, it follows that the ventral fins must be used in making 

 the pits. And since the resistance to the fin is often considerable 

 the necessity of more than ordinary steadiness of the pelvis becomes 

 apparent. As the subabdominal process permits but a small amount 

 of lateral motion, and can be controlled by the bands of muscle 

 attached to the sides of its posterior half, it aids materially in secur- 

 ing the desired firmness. Its position upon the muscular wall of the 

 abdomen effectually prevents anything like a rolling motion of the 

 shaft. 



But while one of the genera of Potamotrygons, Potamotrygon, 

 is well fitted for the digging and for capturing the prey unwary 

 enough to approach the pit, the other, Disceus, is constructed on a 

 different plan. The first has a strong tail and a large caudal spine, 

 which is situated at so great a distance from the body that its wearer 

 can strike an animal on whatever portion of the disk it may happen 

 to touch. Disceus, however, has a greater expanse of disk, a much 



