266 MR. C. TATE REGAN ON THE 



with I 5, pectoral with I 5-6, ventral with I 5. Adipose fin present or absent. 

 Caudal emarginate. 



Rivers of S.E. Brazil ; Amazon and La Plata Systems. 



The genera Hisonotus and Parotocinclus were established on the mistaken supposition 

 that the temporal plates were imperforate in 0, notatus and 0. maculicauda. Exami- 

 nation shows that these two species are true members of the genus Otocinclus with 

 regard to this character. 0. maculicauda is distinguished by possessing an adipose 

 fin, but this feature cannot be considered as of generic importance. Microlepidogaster 

 was established on account of the backward position of the dorsal fin and the numerous 

 ventral plates, but these characters are unimportant, and 0. nigricauda is intermediate 

 between the more typical species and 0. perforates with regard to the former. 

 Another species which proves on examination to belong to this genus is the Bhinelepis 

 lophophanes of Eigenmann. 



Key to the Species. 



I. Scutes not carinate. 



A. Abdomen covered with 3 longitudinal series of plates. 



a. Supraoccipital with median ridge, strongly elevated pos- 



teriorly ; depth of body 4f times in the total length ; 



24 scutes in a longitudinal series ; no adipose fin . . . 1. 0. affinis Stdr, 



b. Supraoccipital without median ridge, not strongly elevated posteriorly ; depth of body 



about 5 times in the total length ; snout less than \ the length of head ; no adipose 

 fin. 

 a. 21-22 scutes in a longitudinal series. 



Diameter of eye nearly 4^ times in the length of head (in specimens 



of 35 mm.) 2. 0. vittatus Rgn. 



Diameter of eye about 3§ times in the length of head (in specimens 



of 40 mm.) 3. 0. vestitus Cope. 



/3. 25 scutes in a longitudinal series 4. O. jiexilis Cope. 



c. Supraoccipital without median ridge, not elevated posteriorly ; depth of body 6-6i 



times in the total length ; snout ^ the length of head. 



An adipose fin 5. O. maculicauda Stdr. 



No adipose fin 6. O. notatus Eigmn. 



B. Abdomen covered with numerous small plates, irregularly arranged. 

 Origin of dorsal a little behind base of ventral, nearer to tip of snout 



than to base of caudal 7.0. nigricauda Blgr. 



4 



Origin of dorsal considerably behind base of ventral, equidistant from 



tip of snout and base of caudal 8. 0. perforatus Eigmn. 



II. Scutes carinate 9-0. lophophanes Eigmn. 



