ps ieee 
THE OSTEOLOGY OF AMIURUS CATUS. 301 
The parts of the dorsal fin may be tabulated as follows :— 
Ist Interspinal .......... Ossicle in front of horizontal plate. 
igi, Rey 2 Sener eens Anterior part of horizontal plate. 
2nd Interspinal .......... Only slightly modified. 
Onl (SOV? (ody GaGa oe The f-shaped bone. 
Slightly modified ; upper portion forms 
the broad surface for support of 3rd ray. 
CS Se The defensive spine. 
The succeeding interspinalia and rays are normal. 
fo] A 
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 hxemal processes of the vertebra, 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 Amiwrus has undergone much modification, 
and has many points of difference from the arches of such forms as 
Salmo and Hsox. It consists of two principal divisions, termed by 
Gegenbaur the primary and secondary shouwlder-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 SCl) is a T-shaped bone, of which the 
upper portion of the transverse limb articulates with the pterotic 
