SENSE ORGANS OF AMIURUS. 379 
the young, which may be connected by a few fibres with that cartil- 
aginous patch which represents the proximal part of the second neu- 
ral arch (Fig. 12, Pl. IV.) 
From the study of Amiurus alone it would be impossible to say 
that the imcus bears the same relation to the cartilaginous neural 
arch of the second vertebra as the anterior process of the stapes does 
to that of the first, but in Catostomus the proximal end of the style- 
like incus contains cartilage and projects from the second vertebra, 
and in Cyprinus the incus has not only articular and anterior, but also 
an ascending process like the stapes of Amiurus. 
A fourth ossicle—the ‘claustrwm ’—assists in forming the wall of 
the neural canal between the ascending process of the stapes and the 
exoccipital. It is somewhat triangular in form, and its apex project- 
ing downwards and backwards fits into the angle between the ascend- 
ing and anterior processes of the stapes. (Fig. 8a, Pl. IV.) It is 
developed in cartilage, and represents the first pair of intercalary neural 
arches which were first pointed out by Geette in the pike, but which 
are present toa greater or less extent in the anterior region of the 
vertebral column in most Physostomous forms. Over the second 
vertebra in the roof of the neural canal, a considerable amount of car- 
tilage persists even in the adult. This does not exhibit any segmen- 
tation, or very little trace of such, (Fig. 10, Pl. 1V.), but probably 
belongs, in part, at least, to the system of intercalary neural pieces. 
For the relation of the dorsal ends of the claustra in the young, vide 
Figs. 9 and 10, P]. 1V. According to Baudelot they meet in the 
middle line of the roof of the neural canal in Szdurus glanis, but this is 
never the case in Amiurus. (Fig. 4, Pl. VI.) Unlike the third and 
fourth vertebrze both the first and second are destitute of transverse 
processes, at least they are almost obsolete in the first and quite so in 
the second. 
The cavum sinus imparis has been referred to above as hollowed 
out in the basi-occipital bone, which also furnishes part of its lateral 
walls. The ex-occipitals furnish the remainder of the lateral walls and 
the osseous roof of the cavum. (Fig. 6, Pl. VI.) This roof is in- 
clined downwards anteriorly, (Fig. 8, Pl. IV. and Fig. 8, Pl. VI.) in 
such a manner as to narrow the aperture of communication between 
the cavum and the cranial cavity. The aperture sutiices, however, to 
27 
