98 BULLETIN OF THE 



or prenasals, possible precursors of the follicles of Savi, evidences of the 

 action of causes tending toward disruption and destruction of the canals. 

 Below the disk the tubules are rather short and are somewhat separated, but 

 not so much so as in Urolophus. 



The great difference of the canal distribution as compared with that ob- 

 taining on the Potamotrygons is evidence to be added to that advanced by 

 the writer in 1877, of the necessity of separating the river Trygons from the 

 species properly belonging to the genus Tseniura. 



Dasybatus. 



Forward on the thoracic region of Dasybatus nudus (Plate XXXIX.) the 

 laterals are nearer to each other than they are above the abdomen. On the 

 shoulder the curve is moderately prominent. There is a pre-scapular branch, 

 and also an area. There are no post-scapulars. The pleurals do not extend 

 much farther out than the basipterygia of the pectorals ; their tubules reach 

 more than half-way to the margin of the disk, and end in small groups of 

 branchlets. The pleurals descend rather close to the forehead. The aural 

 is moderate; the occipitals are longer; and the occipital branches may spring 

 either from the occipitals themselves, or from the laterals, or from both, as 

 may happen, though most often they appear on the first. Posteriorly the 

 cranials converge; the orbital curve is pronounced; and the canals seem to end 

 on the snout. The orbitals cross the pleurals twice; they then go through to 

 the lower face of the disk, more than half the length of the rostrum from the 

 forehead. Their tubules are long and are branched at the ends. One only 

 joins the pleural. 



Under the disk the figures outlined by the tubes are still more character- 

 istic. Within the subrostral loop, in front of the nostril, the pleural makes its 

 appearance. From this point it sweeps out and forward toward the tip of the 

 snout, crossed by the subrostral once and by the suborbital three times. Be- 

 fore reaching the extremity it turns, and, running close along the anterior mar- 

 gin, sends forward a great number of fine short tubules. Near the outer angle 

 of the pectoral it bends across the fin toward the pelvis, in front of which it 

 meets the post-jugular extension. All of the tubules on this surface are along 

 the pleural in front. Four areas are outlined by the suborbital. Only one of 

 them is completely circumscribed by it, from the fact that two of its branches 

 end without connections. With the aid of the pleural, in front, the otherwise 

 open areas one, two, and four are enclosed. The first goes back as far as the 

 nostrils, the second as far as the mouth, the third as far as the middle of the 

 space between the mouth and the gills, and the fourth area ends opposite 

 the second gill opening. Suborbital and subrostral meet at the short orbito- 

 nasal. Angular and jugular are both crooked in irregular flexures. The sub- 

 rostral is very much bent and folded; a prominent loop extends forward in 

 front of the nostril, and another upon the nasal valve. This canal disappears, 

 without visible connections, at the base of the snout. The nasal has not a 



