252 ME. C. TATE EEGAN OjNT 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. Vertebrae 5 + 8+14 (in X. stigmatica). D. I 7-10 ; 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. 



Chcetostomus 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, pi. vi. fig. 2). 



Key to the Species. 



I. Snout with a broad naked margin, without tentacles. 



Dorsal with I 8 rays 1. X. latifrons Gthr. 



Dorsal with I 7 rays 2. X. gymnorhynchus Kner. 



II. Snout with tentacles, the naked margin broad in males, narrow in females. 



A. Dorsal with I 8-9 rays 3. X. dolichoptera Kner. 



B. Dorsal with I 7 rays. 



a. Length of mandibular ramus 3i-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 §— | the length of head . . . . 4. X. lemminckii C. & V. 

 Diameter of eye 6^ times in the length of head (in a specimen of 



145 mm.) ; first dorsal ray |-| the length of head .... 5. X. hopiogenys Gthr. 



b. Length of mandibular ramus 2i-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. 



