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53 MR. C. TATE REGAN ON THE 
by the structure of the skull, which has an outer roof composed of the squamosals and 
superficial portions of the frontals, postfrontals and supraoccipital, which is separated 
by a wide cavity from the brain-case, this cavity being traversed by a few bony stays or 
septa. Vertebre 54+-8+14 (in X. stigmatica). D. 17-10; A. I 3-5. 
Rivers of S. America. 
In all the species the abdomen is naked, the head has no ridges or prominences 
and is flat above, evenly convex at the sides; the scutes are spinulose, not carinate ; 
the supraoccipital is bordered posteriorly by a median scute or pair of scutes and by 
one on each side. X. gymnorhynchus and X. latifrons are distinguished by the 
absence of tentacles on the snout. In the other species the females have a snout with 
narrow naked margin, with a series of tentacles which are in some cases small, 
whilst the males have a much broader naked area, and an additional Y-shaped median 
group of tentacles. The length of the tentacles is subject to great variation in the 
same species ; different individuals of the same size of X. stigmatica have short and 
simple or long and bifid tentacles. 
Chetostomus sericeus Cope (Proc. Ac. Philad. 1871, p. 289) is an insufficiently 
described species from the Upper Amazon, apparently belonging to this genus. 
Hypostomus erinaceus Cuv. & Val. (Hist. Nat. Poiss. xv. p. 510) is founded on 
a small specimen from Chili. The type is preserved in the Paris Museum, but is in 
poor condition, the fin-rays being broken, &c., so that examination only revealed that 
it belongs to a species closely allied to X. cirrhosa, and perhaps identical with it. 
This species has also been described and figured by Guichenot (Gay, ‘ Fauna Chilena,’ 
Pisces, p. 308, pl. vi. fig. 2). 
Key to the Species. 
1. Snout with a broad naked margin, without tentacles. 
Dorsal with @8irays).. > Fee een. |. acne lend. Cat prageaeat 
Dorsal with I'7 rays : . | 4 Sule ee ee ee se Re Ae OY nor hynohusimnen 
II. Snout with tentacles, the naked margin broad in males, narrow in females. 
A. Dorsal with I 8-9 rays ...... =.=. =. =. +. 9&8. X. dolichoptera Kner. 
B. Dorsal with I 7 rays. 
a, Length of mandibular ramus 3}—4 times in the interorbital width, which is 2-2} times 
in the length of head. Length of base of dorsal equal to its distance from posterior 
part or tip of spine of adipose fin. 
Diameter of eye 8 times in the length of head (in a specimen of 
125 mm.) ; first dorsal ray 3—} the length ofhead . . . . 4. X. lemminckii C.& V. 
Diameter of eye 6} times in the length of head (in a specimen of 
145 mm.) ; first dorsal ray {-3 the length of head . . . . 5. X. hoplogenys Gthr. 
6. Length of mandibular ramus 2}-2} times in the interorbital width. Length of base 
of dorsal equal to its distance from anterior part or middle of spine of adipose fin, 
