MUSEUM OF COMPARATIVE ZOOLOGY. 105 



lower face, going between these fins, and there become more sinuous and make 

 broad and sweeping bends. 



At the side of the head, opposite the angle of the mouth, angular, pleural, 

 and suborbital are close together and parallel. Below, the pleural emerges 

 farther back than its point of disappearance on the top. It passes to the side 

 of the face, thence to the pectoral, where, in its outward and its inward course, 

 it traces a pair of lines along the greater part of the anterior border of the fin. 

 Returned from this it runs back nearly parallel with and not far from the 

 jugular extension toward the pelvic region. The tubules of both the anterior 

 lines are directed forward ; of the two posterior lines those of the outer line 

 are extended outward, and those of the inner toward the abdomen, inward. 

 Passing around on the rostral fin, near its border, the suborbital reaches a 

 point on the side of the head, near the corner of the mouth, where it accompa- 

 nies the pleural while making a long loop outward ; coming back from this, it 

 unites at once with the angular. The orbito-nasal is long, curving toward the 

 oral. The nasal itself is neither long nor greatly curved. Two great loops 

 occupy the whole of the subrostral : one turning forward in front of the nostril, 

 and the other backward upon the nasal valve. Both the median and the pre- 

 nasals are short. The latter are not connected with the rostrals. Behind each 

 side of the mouth there is an oral of moderate length, in which the ends extend 

 transversely in opposite directions from a median longitudinal section. 



Rhinoptera (Zygobates) jussieui (Plate LI.). Prominent among the features 

 in which this species differs from the preceding are the increase in the number 

 of tubules on the cranials, the presence of a group of tubules immediately be- 

 hind the orbital on the occipital, the extension of the prelateral branch between 

 the spiracle and the cranial, the shapes of the scapular and the pre-scapular 

 areas, the augmented number of branches on the posterior scapular tubule, the 

 more regular curves in the suborbital and the subrostral, and in the union of the 

 oral across the median line. Besides these there are other particulars of vari- 

 ance, more or less important, as a smaller amount of curvature in the pleural 

 tubules of the ventral series, and greater parallelism in the prenasals, seen on 

 comparison of the drawings. A close relationship of these species is indicated 

 by the many points common to both. 



Dicerobatus. 



Dicerobatus olfersii (Plates LII., LIU.) presents a distribution of the corpo- 

 ral canals that, in the main features, bears much resemblance to that of Mylio- 

 batis, Aetobatus, or Rhinoptera. There is a similar nearly parallel arrangement 

 of two sections of the lower pleural near the front margin of each pectoral, and 

 of two others, closer together, along the basipterygia of the same fin. On the 

 dorsal surface the likeness to Rhinoptera is the greater. At the shoulder there 

 is a single large pre-scapular area. Near the scapular arch the sinuous folds of 

 the pleural are less prominent than in the preceding, but the branchlets of the 

 tubule?: fire even more massed toward the posterior angle of the fin. A post- 



