1891.] *^" [Cope. 



dorsal fia originates above their base, and not behind it, as is the case in 

 0. flexilis. 



Rio Jacuhy, Rio Grande do Sul ; numerous specimens. 



In ihe many specimens of this species and the 0. flexilis in the collec- 

 tion, the adipose fin spine is constantly wanting. 



Plecostomus commersonii Val. 



One adult specimen. A young individual, which I suppose to belong 

 to this species, exhibits the following characters : 



Scuta not connected with each other anterior to the posterior border of 

 the dorsal fin, and consisting of a median ossification only, each scute sup- 

 porting a median comb of bristle-like teeth, which is longitudinal on most 

 of them and is directed obliquely upwards posteriorly on the superolateral 

 series. Besides the central comb, each scute has a shorter one near its 

 superior and inferior extremities. Inferior surface, as far as the vent, 

 without granules or scales. Scuta of lateral line, 28. Traces of a longi- 

 tudinal angle on the posterior part of the supratemporal plate, and of two 

 near together on the posterior part of the occipital. Head marked with 

 coarse granular ridges, which are little marked on the suborbital and in- 

 terorbital regions, and are replaced by fine lines on the anterior part 

 of the occipital plate. Muzzle acuminate oval, viewed from above ; tip 

 naked ; sides without bristles. Occipital bounded posteriorly by a single 

 nuchal plate, which is separated from the basal dorsal fin plate by a second 

 nuchal. Lip large, entire, coarsely tubercular. Teeth about ^f on each 

 side. The humeral angle extends on two plates only. 



Fin radii, D. I. 7; P. I. 6 ; V. I. 5 ; A. 5. The eye enters the length of 

 the head to the apex of the occipital plate six times, three times in the 

 muzzle, and 2.25 times in the interorbital space, which is plane. The 

 base of the dorsal fin is as long as the space from its posterior ray to half 

 way between the adipose fin and the base of the caudal. The pectoral 

 extends a little beyond the base of the ventral, and the ventral to a short 

 distance beyond the posterior ray of the anal fin. The inferior apex of 

 the caudal fin is considerably longer than the superior. 



Total length, 47 mm.; length to base of caudal fin, 3t mm.; length to 

 base of anal fin, 23 mm.; length to ventral, 17 mm.; length to base of 

 pectoral, 9 mm., length of head to apex of occipital plate, 13 mm. Color 

 in alcohol, reddish brown, an obscure dark shade about the base of the 

 dorsal fin. A single row of dusky spots in each membranous space of 

 the dorsal fin. Caudal fin with about three oblique vertical cross-rows of 

 rufous spots. 



The single specimen on which this description is founded is probably 

 young, yet various indications point to its being of small size at n)alurity. 

 The well-tleveloped lip and tubercles are those of a mature fish, and the 

 relatively small size of the eye indicate that little change is to be looked 

 for in the proportions of the head. It is probable that the deficient ossifi- 

 cation of the anterior body plates is a character of immaturity. 



