344 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 

 Thyris,* new genus. 



I feel much hesitation in describing as a member of a new genus this 

 little heterosouie fish, which has all the appearance of being the larval 

 form of some larger si)ecies. Since, however, it has attained almost the 

 size at which one of the associated species begins breeding, and since 

 I am unable to assign it to any genus already described, it seems desira- 

 ble to give it a name which may serve to designate it, at least for the 

 time being. 



Diagnosis. — A genus of heterosome fishes, with soft, transparent, 

 elongate body. Head very short (in the single species contained about 

 5J times in total length of body). Mouth small, toothless. Eyes upon 

 left side, close together, the lower slightly in advance of the upper. 

 Pectoral fin upon blind side shorter and with fewer rays than upon 

 colored side. Ventrals crowded together upon median keel of body, 

 their bases prolonged njion this keel. Dorsal fin commences in front of 

 the eye upon the snout. Dorsal and anal rays simple. Caudal fin sub- 

 sessile, almost confluent with dorsal and anal. Scales very small, thiu, 

 easily detached. Dorsal liue marked, straight. Body translucent, col- 

 orless (except for three longitudinal stripes in the single species). The 

 vertebra can almost be counted through the flesh when the fish is held 

 up to the light, and the arrangement of the gills is clearly visible through 

 the oi^ercular bones. 



Thyris pellucidus, new species. 



The length of the specimen described (No. 20005) is 72 millimeters. 



The height of the body (32) is about one-third of its length (without 

 caudal), the least height of the tail (7) one-fourteenth. The body is 

 thiu, pellucid, larval-like, divided into three longitudinal tracts by de- 

 pressions at the bases of tlie rows of interspinous i^rocesses, as in Gltjp- 

 tocephalus. 



The scales are small, thiu, easily detached (none remain upon the 

 specimen except a few in the lateral line). The number of transverse 

 rows is estimated at one hundred and twenty, the number of rows above 

 and below the lateral line at the widest portion of the body seventeen 

 or eighteen. The scales in the lateral liue are provided with a large 

 central canal. The lateral line is straight on both sides. 



The head is very small; its length (18) contained about five and one- 

 half times in the total length of the body. The eyes are small, protrud- 

 ing, the upper almost perpendicularly above, though i>erh:ips slightly 

 posterior to the lower. The diameter of the eye (2) equals the width of 

 the interorbital space (2) and is double the distance (4) from the snout 

 to the upper eye, that from the snout to the lower eye (3) being inter- 

 mediate. The mouth is small, the shape of the opening being somewhat 

 like that in Solea, the upper jaw being somewhat hook-shaped. The 

 length of the upper jaw (4) is two-thirds that of the mandible (0). 



The dorsal commences on the snout in advance of the eye, ami is com- 



* Etymology : ■9vpig=a, window. 



