1877.] 211 [Garman. 



flattened, wider across the spiracles, not prominent above the disk.. 

 Snout with a small prominence. Eyes small, prominent. Spiracles 

 large, immediately behind the eyes. Nasal valves confluent, attached 

 in the middle, with rounded angles. Mouth small, curved, with five 

 papillae. Sinuation of upper jaws slight. Teeth equal, small, lozenge- 

 shaped on the base, with a transverse ridge bearing three cusps in 

 the young, becoming blunt and losing the lateral cusps in the older, 

 varying in number of series according to age — from twenty- six to 

 forty-eight in upper jaw. Ventrals triangular, with blunt extrem- 

 ities, hinder margin partially serrated. Tail stout, depressed, bearing 

 a large serrated spine at a distance from the base of little less than 

 half the length of the disk, slender posteriorly, compressed into a 

 keel on the upper side, and bearing narrow cutaneous expansions 

 above and below, terminating in a finless thread. The very young, 

 are naked, yellowish, with narrow lines of brown, which, if extended, 

 would form a network. 



Specimens two or three times the size of these are darker colored, 

 with lines more distinct, and a median row of tubercles on the tail 

 anterior to the caudal spine. Medium sized are brownish, the brown 

 lines very dark, polygons indicated in the ground color, the centre of 

 the disk and the top of the tail covered, with asperities, and a row of 

 tubercles on each side of the tail similar to those of the median 

 series. 



Large individuals are of a dark greyish, yellowish or olivaceous 

 brown, with the lines indistinctly defined, the polygons appearing 

 more like rounded spots of lighter color, the entire disk covered with 

 small, harsh scales, the tail furnished with irregular intermediate 

 rows of tubercles, the lateral rows continued on the compressed por- 

 tion behind the spine, and the caudal keel prominent and very rigid. 

 Transverse brown bands separated by white mark the tail at first; the 

 latter are afterward bisected by narrow bands of brown and ulti- 

 mately the white is broken into spots by its encroachments. Marks 

 similar to those on the disk cover the ventrals and retain their dis- 

 tinctness after the body has become uniform. Lower surface whitish ; 

 older specimens acquire a brown margin that widens with age. 



Fourteen specimens collected by the Thayer Expedition. 



Potamotrygon motoro. 



Tceniura motoro Miiller & Henle, Plagiostomen, p. 197. 



Trygon garrapa Mull. & Trosch., Schomb. Reisen, in, p. 642. 

 " " Schomburgk, Fish. Brit. Guiana, n, p. 182, pi. 21. 



