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 8.E. Brazil; Amazon and La Plata Systems. 
‘The genera LHisonotus and Parotocinclus were established on the mistaken supposition 
that the temporal plates were imperforate in 0. notatus and O. maculicauda, Exami- 
nation shows that these two species are true members of the genus Otocinelus 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 O. nigricauda is intermediate 
between the more typical species and 0. perforatus with regard to the former. 
Another species which proves on examination to belong to this genus is the Rhinelepis 
lophophanes of Kigenmann. 
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 42 times in the total length ; 
24. scutes in a longitudinal series ; no adipose fin. . . 1. O. affinis Stdr. 
4. 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 44 times in the length of head (in specimens 
of 35mm.) . . 5 . . . . . » & O. vittatus Ken. 
Diameter of eye about 32 times in the ein of Pas fn specimens 
of 40mm.) . . . . . 6 2 = + + + + + + + « « « 8. O. vestitus Cope. 
A. 25 scutes in a longitudinal series. . . . . - . . 4& Oz flexilis Cope. 
c. Supraoccipital without median ridge, not elevated posteriorly ; depth of body 6-6} 
times in the total length; snout 4 the length of head. 
An adipose fin. . . . 2 1 1 ee ee ee ee es + + 5. O. maculicauda Stdr. 
WNoadiposefin.', . 2. . « - Se ies Ge... 6 eo 6 motatas Migr, 
B. Abdomen covered with numerous small plates, irregularly arranged. 
Origin of dorsal a little behind hase of ventral, nearer to tip of snout 
than to base of caudal. . . . . : see . . . 7%, O. nigricauda Blgr. 
Origin of dorsal considerably behind base of Satvall Be nidlistart from 
tip of snout and base ofcaudal. . . «© + + s+ + 8 ee 8. O. perforatus Kigmn. 
II. Scutescarinate. . . . . . . + © © © © «© + «© » 9 O. lophophanes Eigmn. 
