230 L. Agassiz on the Ichthyological Fauna of Western America, 
tween the extremity of the intermaxillary and upper maxillary 
bones. In addition to this peculiarity which our Mylocheilus has 
in common with the Eastern American Leucisci, our fish has a 
horny sheath surrounding both the upper and lower jaw, so that 
we are led to consider it without hesitation as a new generic type, 
peculiar, as far as we know at present, to the Northwestern coast 
of America. A close examination of the pharyngeal system of 
teeth fully sustains the impression, first received from the exam- 
ination of the jaws, that Mylocheilus constitutes a distinct genus in 
the family of Cyprinids ; for its teeth differ entirely from those 
of the common Cyprinide known at the present day. Far from 
in one main row of five, in which the anterior teeth are the largest, 
and the posterior are much smaller and somewhat compressed 
and hooked. Oné tooth is wanting in the left row, but its base 
of insertion shows that it has recently fallen off. In addition to 
those teeth, there is above on each side one small tooth similar 
in form to the common small tooth of Leucisci; underneath on 
each side, one or two immature teeth sticking in the gum. e 
it not for the circumstance that the two arches of the phary ngeal 
bones are free and movable upon their symphysis, one might sup- 
pose this apparatus to have belonged to some Labroid Fish ; 80 
great is the resemblance of its rounded teeth to those of that fam- 
ily. The horny plate against which this apparatus moves U 
the basilar bone is ovate, lanceolate. The arches of the gills are 
provided, on their inner margin with tufts of small teeth form- 
ing canals which alternate between the adjacent arches, the upper 
ones of the first arch only consist of strong hooks. The nostrils 
are large ; the anterior, tubular; the posterior crescent-shaped. 
Water-pores seem to be fewer than in Acrocheilus. I perceive 
only those larger ones which follow the shoulder bone, the mas- 
toid, the preoperculum, the suborbital, and the lower jaw. The 
branchiostegal membrane unites in advance of the humeral bone, 
so that the branchial fissure does not extend to the side of the 
hyoid bone. he pectorals are large, and longer than the ven- 
trals, the latter placed nearer to the anus than to the pectorals. 
The dorsal begins in advance of the ventrals which are opposite 
the middle of that fin. There is one strong simple ray, 19 the 
anterior margin of the pectorals, followed by seventeen branching 
and articulated rays, the last of which however are simple. 
ventrals consist of one simple ray and eight branching one’ 
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