THE OSTEOLOGY OF AMIURUS CATUS. 301 



The parts of the dorsal fin may be tabulated as follows : — 



1st Interspinal Ossicle in front of horizontal plate. 



1st R.ay Anterior part of horizontal plate. 



2nd Interspinal Only slightly modified. 



2nd Ray The H -shaped bone. 



3rd Interspinal \ ^^J modified ; upper portion forms 



L ( the broad surface for support of 3rd ray. 

 3rd Ray The defensive spine. 



The succeeding interspinalia and rays are normal. 



VII.— THE ANAL FIN. 

 The anal fin is constructed on the normal type, consisting of 21-22 

 rays, osseous at the base, but horny a slight distance outward. The 

 interspinalia are completely osseous, and are not quite regular in 

 their arrangement to the haenial processes of the vertebrse, two 

 interspinalia occurring at irregular intervals in the space between 

 two processes. 



VIII.— THE CAUDAL FIN. 



The caudal fin is also normal. The rays here are also osseous at 

 the base. Those in the centre are shorter than those above and 

 below, and a few short rays run forwards a short distance above and 

 below upon the body. 



The adipose fin, containing no osseous skeleton, belongs more pro- 

 perly to the tegumentary system. 



IX.— THE PECTORAL ARCH AND FIN. 



The pectoral arch in Amiurus has undergone much modification 

 and has many points of difference from the arches of such forms as 

 Salmo and Esox. It consists of two principal divisions, termed by 

 Gegenbaur the primary and secondary shoulder- girdles. In the 

 majority of the Teleosts the latter is much the larger, the former 

 forming as it were a mere appendage to it. In Amiurus this is not 

 exactly the case, for the primary girdle, or at any rate an extension 

 of it, forms a large part of the pectoral arch. All parts of the arch 

 are completely ossified, and considerable modifications are present in 

 relation to the peculiar articulation of the fin ray. 



The secondary shoulder-girdle consists of two pieces. The upper 

 or supraclavicula (Fig. 1 SCI) is a T-shaped bone, of which the 

 .upper portion of the transverse limb articulates with the pterotic 



