1889.] 



Contributions to the A natomy of Fishes. 



317 



general features of structure or the more special points involved in 

 the mode of formation and relations of the " cavum sinus imparis," 

 or of its bilobed backward prolongation, the " atria sinus imparis." 

 The uniformity in the latter respect is so marked, that a description 

 of those structures as they occur in any one normal Siluroid will 

 practically apply to all the others. 



As regards the internal ear, the condition of many of our specimens 

 was such that our observations were necessarily somewhat incomplete. 

 The condition of the membranous labyrinth, and its relations to the 

 cavum sinus imparis and to the atrial cavities, were investigated in 

 a large number of Siluridse normales but with the purely negative 

 result that we could detect no variations of any importance from the 

 arrangement of these structures, already described for Amiurus catus 

 by Ramsay Wright, and for Silurus glanis by Weber. In all cases 

 we found a transversely disposed ductus endolymphaticus connecting 

 the two sacculi, and, attached to the ductus, a median pear-shaped 

 sinus endolymphaticus projecting backwards into, and almost com- 

 pletely filling, the " cavum sinus imparis." 



With the exception of the intercalarium, the Weberian ossicles 

 exhibit but little variety in shape or in their relations to one another 

 or to the atrial cavities and air-bladder. The variations in the 

 condition of the tripus relate principally to the degree and shape of 

 the curvature of its posterior or crescentic process. In some genera 

 (Auchenipterus, Oxydoras) the crescentic process is almost straight; 

 in others almost hook-shaped (Plotosus) ; and between these extremes 

 the process may exhibit almost every degree of curvature. A ventral 

 ridge on the root of the crescentic process, to receive the insertion of 

 a slip of fibres from the adjacent anterior wall of the bladder, is very 

 generally present, and varies in size according to the thickness of the 

 walls of the bladder. In some Siluroids (Macrones, Liocads) the 

 outer convex margin of the process may be increased for the purpose 

 ol fibrous attachment by the addition of an outwardly directed heel- 

 like process. The articular process of the tripus is usually distinct 

 from the complex centrum, with which, however, it articulates at the 

 bottom of a deep pit-like depression. It is very rare, as in one genus 

 (Auchenipterus), for the process to be flexible and elastic, and directly 

 continuous by an oblique origin with the anterior part of the neural 

 arch of the complex vertebra like the adjacent and similarly elastic 

 root of the " elastic-spring-apparatus." The proportional lengths 

 of the anterior and crescentic processes vary somewhat in different 

 forms ; generally, the two processes are of approximately equal length, 

 but when otherwise it is the anterior which is the longer. 



The intercalarium varies greatly in development. Usually a small 

 osseous nodule imbedded in the interossicular ligament, the inter- 

 calarium may, in addition, be prolonged therefrom as a horizontal 



