i8 Memoirs of the Indian Museum. [Vol. II, 



each of the more important measurements might be taken. So far as the Trygonidse 

 are concerned I have measured the length of the disk from the tip of the mouth to the 

 base of the tail and have not included the length of the pelvic fins. A more exact 

 measurement might be obtained on the ventral surface, but for diagnostic purposes 

 the one adopted is sufficiently accurate. The breadth of the disk has been measured 

 across the back with a tape, not from point to point. This method gives a more ac- 

 curate idea of the bulk of a flat fish than that of measuring from point to point, but of 

 course it makes the deeper species, such as Trygon microps, appear to have a larger 

 expanse of pectoral fin than is actually the case. The snout has been measured from 

 a point just in front of the eyes, the interorbital space from the inner margin of one 

 orbit to that of the other, the tail from the posterior end of the disk. 



The genera of Trygonidae are, for the most part, sufficiently defined in the above 

 key, so far as the Indian forms are concerned. Urolophus, which is recorded from Java 

 and may very probably occur within our area, is distinguished from Trygon by the 

 possession of a rayed caudal fin and, in some species, a rudimentary dorsal one. 



Genus Trygon, Adanson. 

 Disk distinct from tail (which is always powerful), more or less flattened, never very 

 much broader than long. No rayed caudal or dorsal fins ; at least one serrated 

 caudal spine, except in abnormal specimens ; a rayless caudal fold sometimes 

 present on the dorsal or ventral surface of the tail, or on both surfaces, but 

 never much deeper than the tail itself. Teeth as a rule flat with a transverse 

 ridge, rarely bearing a sharp spine. 



In the genus Trygon, an important specific character consists in the number, 

 proportion, and outline of the cutaneous processes on the floor of the mouth behind the 

 cutaneous fold that hangs down from the roof. These can only be investigated pro- 

 perly by dissecting out the mouth. Although they are not absolutely constant in any 

 one species, the evidence they afford as to the distinction between allied species such 

 as T. uarnak and T. favics is very valuable, for the differences they display are often 

 strongly marked in allied species. The character of their variation is best illustrated by 

 examples. In T. gerrardii {plate ii, fig. 6) there are usually four processes, the two central 

 ones being much stouter and longer than the two lateral ones. As a rule the central 

 ones are pointed, but in one specimen dissected (plate ii, fig. 6a) they are blunt and 

 fringed at the tip. In another specimen the two lateral processes are absent. In what 

 seems to be a normal specimen of T. microps there are three central processes joined 

 together at the base so as to form a deeply serrated ridge, but in three out of the five 

 individuals I have dissected the arrangement of these processes is not quite sym- 

 metrical, while in one there are four central processes with traces of a fifth. 



Key to the Indian species of Trygon. 

 1= Large species (adults over 40 cm. across the disk, young over 15 cm.) 

 as a rule with a single large serrated spine ^ on the tail ; cutaneous 

 caudal fold, if present, inconspicuous. 



There is usually a second, smaller one concealed under the first. 



