RHOMBOSOLEIN,E 415 



Depth of body about twice in the length, length of head 3J to nearly 4. Upper 

 profile of head distinctly convex. Snout longer than eye, diameter of which is 4-J 

 to 5 in length of head and about twice interorbital width ; anterior margins of eyes 

 level or lower a very little in advance of upper, which is separated from edge of head 

 by a space equal at least to its diameter. Maxillary not reaching anterior edge of 

 eye, length on ocular side 3J to 3^, on blind side 2f to 2J in that of head ; lower jaw 

 not projecting, about 2 J in head; lips rather feebly fringed. 9 to 11 gill-rakers 

 on lower part of anterior arch. About 115 scales in lateral line. Dorsal (60) 61-67 '■ 

 first ray with its upper and lower edges fringed ; the remainder of the anterior rays 

 deeply forked, free from membrane distally ; highest rays ^ to f length of head. 

 Anal (42) 43-45. Pectoral of ocular side with 11 to 13 rays (8 to 10 branched), length 

 I J to I J in that of head. Anterior ray of pelvic of ocular side inserted below eye. 

 Caudal with 16 rays (12 branched), rounded ; caudal peduncle more than twice 

 as deep as long. Pale brownish, everywhere covered with small white spots ; some 

 larger white spots at upper and lower edges of body. 



Type. — Vienna Museum. 



Distribution. — South-east coast of South America, from Rio Grande do Sul 

 to San Mathias Bay. 



Specimens Examined : 



1 (125 mm.), skin. Bahia Blanca, Patagonia. Cambridge Mus. (" Beagle "). 



2 (188, 215 mm.). San Mathias Bay, ,, Mus. Comp. Zool. 

 I (250 mm.). 



Attains to a length of 10 or 1 1 inches. 



The function of the curiously modified first ray of the dorsal fin is not known, 

 but it may be sensory. The groove in which it lies penetrates the whole thickness 

 of the head, being bordered internally merely by the skin of the ocular side. According 

 to Steindachner, the ray is fixed to the base of the cavity by a muscular membrane, 

 and only exhibits a moderate degree of mobility. Towards its base the ray broadens 

 out to form two small plates, which clasp the anterior end of the first strongly developed 

 basal bone of the dorsal fin. 



Genus 37. PSAMMODISCUS. 



Psammodiscus , Giinther, 1862, Cat. Fish., iv, p. 457 [Psammodiscus ocellatus, Giintherj. 



Related to Oncopterus. Eyes separated by a low, narrow ridge. Cleft of mouth 

 nearly vertical, the jaws stronger on ocular side ; teeth uniserial or in a very narrow 

 band. Gill-rakers rather short, more numerous ; lower pharyngeals very nairow, 

 not approximated, each with 2 or 3 rows of small, pointed teeth ; branchial septum 

 entire. Dorsal fin commencing in front of nostrils of blind side ; first ray not specially 

 modified ; most of the rays scaled, posterior rays mostly forked distally. First 

 ray of pelvic fin of blind side opposite fourth of that of ocular side ; base of moderate 

 length. Caudal peduncle very short. Lateral line with a distinct supratemporal 

 branch, but with no other accessory branches. 



A single species. 



I. PSAMMODISCUS OCELLATUS, Giinther. 



Psammodiscus ocellalus, Giinther, 1862, Cat. Fish., iv, p. 457. 



Depth of body if to ij in the length, length of head 3J to 35. Upper profile 

 of head distinctly convex. Snout shorter than eye, diameter of which is 3J to 4 in 

 length of head and about 5 times interorbital width ; lower eye a little in advance 

 of upper, which is separated from edge of head by a space equal to at least i its diameter. 

 Maxillary extending to below anterior edge, or anterior part of eye, length on ocular 

 side 2i to 2§, on blind side 3 J to 35 in that of head ; lower jaw not projecting, 2 J to 

 2 J in head ; lips not fringed. 15 to 18 gill-rakers on lower part of anterior arch. 



