206 MR. C. TATE REGAN ON THE 
Plecostomus flavus Vaill. Bull. Soc. Philom. (7) iv. 1880, p. 155. 
Plecostomus plecostomus FKigenm. & Eigenm. Proc. Cal. Ac. (2) i. 1888, p. 168; Occ. Pap. Cal. 
Ac. i. p. 406 (1890). 
Plecostomus seminudus Figenm, & Eigenm. t. ec. p. 169. 
Plecostomus boulengeri Viigenm. & Kennedy, Proc. Ac. Philad. 1903, p. 502. 
Depth of body 43-5 times in the total length, length of head 3-5} times. Depth 
of head 14 times in its length, breadth of head 1} times, length of snout 12 times, 
diameter of eye 54-94 times, interorbital width about 23 times. Length of mandibular 
ramus about 3 times in the interorbital width. Snout ovate, more or less narrowed 
anteriorly ; supraorbital margins raised; supraoccipital with moderately elevated 
median ridge; temporal plates carinate. Scutes spinulose, the anterior and upper 
scutes carinate, 25-26 in a longitudinal series, 7-S between dorsal and adipose fin, 
13-14 between anal and caudal. Supraoccipital entirely bordered posteriorly by a 
single scute. Lower surface of head and abdomen, in the adult, almost completely 
covered with small granular scales. D. I 7, the first ray about as long as the head, the 
Jast 3-5 as long; length of base of dorsal equal to its distance from posterior 4 of spine 
of adipose fin. A. I 4. Pectoral spine extending well beyond base of yentrals. 
Caudal emarginate, the inner rays 5—} the length of head, the outer produced. Caudal 
peduncle 5-3} times as long as deep. In the adult, numerous dark spots on head, 
body, and fins, those on the head smallest, those on the dorsal in 2 rows on each 
interradial membrane. In the young, spots on the head and body larger and fewer, 
fins with 4—6 transverse bars. 
Total length 350 mm. 
‘Twelve specimens: R, Amazon; Guiana; Venezuela; Trinidad. 
2. PLECOSTOMUS COMMERSONII. 
Hypostomus commersonii Cuy. & Val. Hist. Nat. Poiss. xv. p. 495 (1840) ; Val. Voy. d’Orbigny, 
Poiss. pl. vii. fig. 2 (1847). 
Plecostomus commersonii Giinth. Cat. Fish. v. p. 232 (1864). 
Plecostomus spiniger Hensel, Arch. Nat. i. 1870, p. 73. 
Plecostomus limosus Bigenm. & Eigenm. Proc. Cal. Ac. (2) i. 1888, p. 167, and Occ. Pap. Cal. Ac. 
i. 1890, p. 405. 
? Plecostomus aspilogaster Cope, Proc. Am. Phil. Soc. xxxiii. 1894, p. 100, fig. 14. 
Depth of body 53-53 times in the total length, length of head 33-53 times. Depth 
of head 13 times in its length, breadth of head about 1} times, diameter of eye 8-13 
times, length of snout 1% times, interorbital width 24-25 times. Length of mandibular 
ramus 23-3} times in the interorbital width. Snout rather broad, rounded anteriorly ; 
supraorbital edges slightly or moderately raised; supraoccipital with median ridge ; 
temporal plates more or less distinctly carinate. Scutes spinulose, mostly carinate, 
28-50 in a longitudinal series, 8-9 between dorsal and adipose fin, 14-15 between 
anal and caudal. Supraoccipital entirely bordered posteriorly by a single scute. Lower 
