98 L. Agassiz on the Ichthyological Fauna of Western America. 
in addition to those exhibited by the structure of the rays them- 
selves. As faras [ can ascertain, it has been admitted among 
ichthyologists, that the change of form in the fins arose from the 
rays being brought close together, or stretched asunder ; I find, at 
east, no mention in any description, of rays overlapping each 
other, as [ have shown it to be the case among Balistidee, and as 
is also the case among many others. In the Seomberoids, for 
instance, the rays of the vertical fins are remarkably spreading 
when contrasted with those of the Balistide, or those of the genus 
Acrocheilus described here. The ventrals are rather large, some- 
what similar in their rounded form and the thickness of their 
rays, to the ventrals of the genns Tinea; the first ray especially 
is thick and simple. It is fellowed by eight artictlated rays. 
he pectorals are also somewhat rounded, but not so much as 
the ventrals, the upper angle projecting more, "Their first ray is 
also thick and simple, and is followed by sixteen articulated rays 
gradually tapering, the last of which, however, are simple. In 
its general form, the fish upon which this new genus is founded, 
has considerable resemblance to the European Chondrostoma Na- 
sus, and [ should not be surprised at all if, upon a superficial ex- 
amination, it had been identified with it, notwithstanding the 
generic and specific differences, to which I have already alluded. 
‘The scales. however, present a striking difference. They have 
not, as iu Chondrostema, the ordinary type of Leuciseus, but re- 
semble rather the scales of Barbus in their elongated form, their 
small size, their many radiating furrows diverging in every direc- 
tion, and their ornamental pigment cells which are especially nu- 
merous along the posterior margin. ‘Ihe ceutre of radiation is 
far in advance of the centre of form. ‘The lateral line arises above 
the posterior and upper angle of the operculum, and is first slightly 
bent downwards, so that it follows in its conrse upon the side, @ 
direction vearer the abdominal margin than the back ; but npon 
the tail it is strictly upon the middle of the side. ‘The tubes of 
these scales arise in the middle of the anterior field, aud taper 
towards the middle of the posterior field, where they terminate. 
The scales along the back. npon the neck, between the pectorals, 
aud along the lower margin of the abdominal cavity, are much 
smallerthan upon the middle of the sides. There is a naked 
space behind the pectorals in which the muscular swelling of the 
base of that fin is received, when the fin is bent backwards. 
There is also a narrow smooth space above the ventrals; along 
the base of the dorsal and anal the scales do not extend quite to 
the base of the rays, but upon the candal they cover their base 
completely and even extend somewhat along the sides of the 
middle rays. 
Water pores besides those of the lateral line, are very distinct 
upon the neck in advance of the scales. The whole surface of 
the skin covering the skull seems also to be perforated by a set 
