41 



base, and almost parallel to the axis of tlie fish. The outer part of the fin-raj^ formed 

 of the two united lateral halves bifurcates in an antero-posterior direction into two 

 divergent branches. Thus the outer half of the fin-ray consists of two diverging 

 branches both in the plane of the fin, one anterior and one posterior, but each of these 

 is made up of two lateral halves glued as it were together lengthwise. Thus if a fin- 

 ray is separated, and a knife passed between the diverging right and left legs of the ray, 

 the whole ray can be easily divided into a right half ray and a left half ray, and each 

 of these is forked at its outer half length into two branches. The basal portion of the 

 fin ray is solid, but the outer five-sixths, including the two forked branches, consist of 

 a series of short cylindrical bony pieces united by flexible membrane. Thus the outer 

 part of the fin-ray resembles somewhat the series of vertebral centra on a small scale 

 and without the spines, if we suppose the series of centra after being single for a 

 certain length to branch into two series having the same structure. 



The tail fin-rays resemble those of the dorsal and anal fins in general structure, but 

 are more branched : in them each of the two original branches divides again into two, 

 and some of these again into two : the branching of the ray is always dichotomous, 

 that is, a branch when it divides splits only into two smaller branches, but of the four 

 secondary branches all do not alwa3's divide : thus the fin-ray terminates in seven or 

 eight branches spread out like a fan. The caudal fin-rays are not in connection with 

 either interspinous bones or cartilaginous nodules, but are articulated directly to the 

 terminal bones of the vertebral column, and to the spines of the last vertebra. 



The skeleton of the paired fins is closely related to that of the branchial region and 

 will be described in connection with it. 



The Jaws and Branchial Arches. 



We have now to consider the bones of the jaws and branchial arches, and other 

 bones connected with the skull. The more superficial of these bones are shown in 

 Figs. 1 and 2, PI, XI. The bones of the right side. Fig. 1, difler considerably in size 

 from those on the left, although in their relations the bones of the two sides correspond 

 to one another. The bones of the right side are as follows. The two sockets in the 

 side of t)ie cranidl portion of the skull, in the pterotic and sphenotic bones, contain 

 the two rounded heads of a flat elongated bone which extends downwards and 

 somewhat forwards. This is the hyomandibular, hm. To the lower end of this bone is 

 attached a system of flat bones which projects horizontally forwards into a smooth 

 knob with which the lower jaw is articulated, and sends oS" upwards and forwards a 

 flat band of bone, the end of which is again attached to the skull at the side of the 

 vomerine projection. At the posterior border of the hyomandibular and the system 

 of bones just mentioned is a crescent-shaped bone, which is firmly attached to the 

 others and acts like a splint, binding them rigidly together. This is the preopercidar, po. 

 The separate bones of the system connected with the lower end of the hyomandibular 



G 



