426 THE PLAGIOSTOMIA. 



in the circular disk, the absence of a process behind the eyelid, the presence of a 

 serrated caudal spine, the absence of tubercles on the base of the tail and in 

 having a slight prominence on the anterior margin of the spiracles. It agrees 

 with E. spinicauda in position of eyes, mouth, and spiracles near the anterior 

 borders and in shapes of tail and fins. The short tail with the caudal spine near 

 the disk separates this form from the Potamotrygons. It appears to be an in- 

 termediate between Potamotrygon and Disceus that may be expected in future 

 collections. At present it is placed among the doubtful species. Miiller and 

 Troschel say it belongs in the group of Trygons without fins on the tail; they 

 also say the complete tail is two thirds of the length of the disk. According to 

 these authors the teeth occupy only the middle third of the mouth and are 

 arranged in twelve longitudinal rows; this and the proportions they give their 

 specimen make it appear to belong to the genus Disceus, but they say nothing of 

 a spiracular process which, if it had been present, they would hardly have over- 

 looked. 



Disceus. 



Disceus Garmax. 1S77. Proc. Bost. soc. nat. hist., 19, p. 208. 



Disk subelliptical, longer than broad. Nostrils, mouth, eyes, and spiracles 

 small, near the middle of the disk. Spiracles with a strong process on the outer 

 border. Ventral fins triangular, covered by the pectorals; pelvis with a long 

 prepelvic process from the middle. Lateral system of the lower surface retiform. 

 Tail slender, with a serrated spine near the disk and with cutaneous folds above 

 and below the filiform portion. 



Disceus thayeri, sp. now 

 Plate 34; Plate 54, fig. 2 (pelvis); Plate 57, fig. 2 (heart); Plate 70, fig. 3 (skeleton). 



Disceus stnmgyloplerus Gakman, 1877, Proc. Bost. soc. nat. hist., 19, p. 208; 1888, Bull. M. C. Z., 17, p. 



95, pi. 3.5 (non Schomburgk). 

 tTrygon aiereba Mullee cV IIexle, 1841, Plagios., p. 100 (non p. 161, non Marcgrave). 



Disk elliptical, truncate or concave in front. Head small, at the end of the 

 anterior third of the disk. Eyes very small, far apart, prominent. Spiracles 

 small, with a strong process on the outer margin. Mouth small, regularly 

 arched forward, with two papillae at the bottom, less distinct in large specimens. 

 Teeth small, bands extending halfway from symphysis to angles, in sixteen to 

 nineteen rows on each jaw, flat, triangular, and sharp-edged on the crown. 



