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 incus 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 pi-oximal end of the style- 

 like incus contains cartilage and projects from the second vertebra, 

 and in Cy primes the incus has not only articular and anterior, but also 

 an ascending process like the stapes of Amiurus. 



A fourth ossicle — the 'claustrum ' — assists in forming the wall of 

 the neural canal between the ascending process of the stapes and the 

 exoccipital. lb 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, PI. IV.) It is 

 developed in cartilage, and represents the first pair of intercalary neural 

 arches which were first pointed out by Gcette in the pike, but which 

 are present to a 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, PI. IV.), 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, PI. IV. According to Baudelot they meet in the 

 middle line of the roof of the neural canal in Silurus glanis, but this is 

 never the case in Amiurus. (Fig. 4, PI. VI.) Unlike the thrrd and 

 fourth vertebra? 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 suffices, however, to 

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