42 



are as follows. Directlj^ attached to the end of the hyomaudibular is seen the end of 

 a bone which passes downwards beneath the preoperculum out of sight, and is 

 connected with the system of branchial arches which occupy a deeper position. This 

 is the stylohyal. Anterior to this is a flat bone which projects into the quadrate in its 

 under surface, this is the symplectic, s. Dorsal to this is a flat somewhat concave bone 

 with a triangular outline, the inetapterygoid, mt. The triangular bone, which is at the 

 anterior end of the series, and which supplies the rounded head to which the lower 

 jaw or mandible, m., is articulated, is the quadrate, q. Eunning upwards from the 

 quadrate dorsally are a pair of splinter-like bones, the anterior of which is the 

 pterygoid, pt., and the posterior the mesopterygoid, ms. Connecting these with the 

 skull at the side of the vomerine process is the palatine, pa. It will be seen that all 

 these bones are somewhat larger on the left side, Fig. 2, than on the riglit, Fig. 1. 

 The superiority is especially well marked in the pterygo-palatine bar, which is broad 

 and strong on the riirht, narrow and delicate on the left. 



Attached to the posterior edge of the hj'omandibular and 23reopercular is the oper- 

 culum, which is chiefly composed of the three bones,o.,io., so. : these are the opercidar, o., 

 which is articulated to a knob projecting downwards on the posterior side of the 

 head of the hyomaudibular, the inter opercular, i.o., and the sub-opercidar, s.o., which 

 are only held in their places by the fibrous tissue of the operculum. The lower jaw 

 is called the mandible, which consists of three bones, the articular, angtdar, and dentary, 

 fitted and firmly united together : the upper jaw consists of two bones, the maxilla, x, 

 and the premaxilla, p.,x. All these bones are much smaller on the right or upper side 

 of the head than on the left or lower ; this is especially true of the maxilla and 

 premaxilla which, as the figures show, are almost rudimentary on the right side, and 

 very large on the left. On the left side the maxilla is entirely excluded from the edge 

 of the lip ; on the right side its posterior process forms part of that edge. The 

 mandible on the right side has the form of an elongated bar, on the left side it is a 

 broad triangular stout plate, the dorsal edge of which is strongly convex and bears a 

 patch of rod-like teeth, biting against a concave larger patch of similar teeth in the 

 premaxilla. On the left side the end of the premaxilla articulates with a knob on the 

 mandible, an arrangement which makes the bite much more effective ; on the right side 

 there is no such articulation. The two mandibles are firmly united together anteriorly 

 in what is called the symphysis of the mandibles. The anterior ends of the maxillte 

 and premaxillae are all firmly united together and to the lower side of the mesethmoid 

 bone by fibrous membrane. 



If we now dissect away all the bones above described except the hyomaudibular, 

 we find the bones of the branchial apparatus lying beneath them as shown in Plate XI, 

 Fig. 3, from the right side. The gills themselves are vascular fringes which project 

 outwards and backwards from a series of bars ; between these are the clefts by which 

 the water passes from the cavity of the throat to the gill chamber and so to the exterior. 

 These bars are called the branchial arches, from their curved shape, and each bar is 



