90 L. Agassiz on the Ichthyological Fauna of Western America. 
. 
Hylomyzon, Agass. 
The name of this genus isa mere translation of the vernacu- 
lar name of ils type, the Mud-Sucker of the West, framed in im 
tation of Petromyzon, but expressing its habits of living in the 
mud. The body is stout and heavy in front, and tapers off rap- 
idly from the shoulders towards the tail ; behind the dorsal it is 
nearly cylindrical in form 
The short quadrangular head is broad and flat above, its sides 
are vertical. The eyes are of moderate size, and elliptical in 
m; the superorbital ridges are elevated above the general level 
of the head. The mouth is inferior, and encircled by broad, 
fleshy lips, which are covered by small granules or papilla. The 
lewer lip is bilobed. ‘The dorsal is over the ventrals, and nearer 
the head than the tail; its height and length are nearly equal. 
The pectorals and ventrals are broad and rounded, the anal fiv is 
slender aud reaches the caudals. The scales are largest on the 
aiterior portion of the body. They are slightly longer than high; 
the ornamental conceutric ridges of the posterior field are broader 
and farther apart than those of the lateral and anterior fields ; no 
radiating furrows upon the lateral fields; those of the — 
and posterior fields rather remote, about equal i in bumber. Tube 
of the lateral line arising from the centre of radiation. 
The teeth are compressed, so that their sharp-edge projects in- 
wards ; at the same time they are slightly arched inwards and in- 
serted obliquely upon the pharyngeal bones. They increase 
gradually iv size and thickness from above downwards. The 
more projecting, the inner point however projecting more than 
the outer one. Fig. 7, a, represents the right pharyngeal of Hy- 
w]e. 
- lomyzon nigricans, b and c¢, one of the lower teeth from two sides, 
and d one of the middle teeth in profile. 
_'There is no species in the whole tribe of Catostomi which has 
described under so many names as the type of this genus. 
It was first described by Lesueur under the name of Cat. nigri- 
caus, from specimens obtained in Lake Erie. At the same time 
he described specimens from Pipe Creek, Maryland, under the 
