2 OS CEYLON PEARL OYSTER REPORT. 



small and are apparent only on a microscopical examination of the skin ; the ctenoid 

 scales along the fin margins are just indicated ; and the ratio of the height of the 

 body to the length (exclusive in both cases of the fins) is 073 : 1. 



Five specimens differ slightly from the above. The general surface of the body is 

 pigmented (brownish in spirit) ; the interorbital space is nearly equal to the major 

 diameter of the eye ; the ctenoid scales along the fin margins are very evident ; and 

 the ratio of the height to the length of the body in the largest is - 69 : 1. 



The single large specimen (Plate II., fig. 3) is 4 "7 inches in total length. It is a 

 female with ripe ovaries. The interorbital space is nearly equal to the major diameter 

 of the eye. The ratio of the height of the body to the length is 0'65 : 1. The length 

 of the head is contained 4 times in the total length, the tail 5|- times ; the length of 

 the maxilla is contained 3j times in the length of the head. The upper ray of the 

 pectoral fin on the ocular side is elongated and is nearly -J of the total length. The 

 scales are cycloid, except for several rows along the fin margins on the ocular side, 

 which are strongly ctenoid. They are very small, and there are about 80 in the 

 lateral line. The jaws are slightly, and the teeth markedly, asymmetrical. The cleft 

 of the mouth is twisted dorsally on the blind side and on that side the teeth are 

 strongly developed, there being several rows in both jaws. On the ocular side there 

 are also several rows of less strongly developed teeth. Teeth are absent on the vomer 

 and palatines. 



The radial formula is : D. 96 ; A. 69 ; P. 8 ; V. 6 ; C. 17. 



The dorsal fin begins on the snout on the blind side. The positions of the ventral 

 fins are markedly asymmetrical. 



The anus is situated on the blind side and the urinary opening on the ocular, in 

 both cases some little distance from the ventral margin of the body. 



Alcock's definition of the genus must then be amended. The body is sub-circular 

 (in the young) ; jaws and dentition asymmetrical ; length of maxilla more than J of 

 the length of the head ; eyes on the left side separated by a broad concave space ; 

 dorsal fin commencing on the snout ; its rays and those of the anal simple ; scales 

 small, those on the general body surface cycloid, but along the bases of the dorsal and 

 anal fin strongly ctenoid ; lateral line with an angular elevation above the pectoral 

 fin ; the latter with its upper ray (sometimes) elongated. 



The genus is closely allied to Rhomboidichthys, and Psettyllis ocellata to Rhombvid- 

 ichthys ocellatus, Agassiz. Some species of Rhomboidichthys, i.e., R. podas and 

 R. mancus, possess spinous scales along the fin margins, and in many others the 

 pectoral fin may be elongate. Psetti/lis, however, seems to differ in the following 

 characters: (1) cycloid scales on the general surface of the body and (2) the 

 asymmetrical jaws and dentition. 



It is not improbable that Psettyllis pelluci-da may be the young of some form 

 belonging to another genus ; at any rate, the characters given by Alcock are 

 probably not those of the adult. 



