304 PKOCEEDINGS OF THE CANADIAN INSTITUTE. 



anteriorly, is similar in appearance, but stouter. Distally, like the 

 basale, it supports the tin rays, but proximally it articulates with the 

 upper surface of the anterior extremity of the spicule-like bridge. 

 Between the distal ends of this, which is a radial and the basale, is 

 a small cartilage, embraced by the fin-rays. The next element, 

 anteriorly, is a large cartilaginous nodule, articulating with the 

 extremity of the rod-like process of the coracoid, and supporting the 

 fin rays. It probably represents another radial. Huxley's meso- 

 pterygial basale is here, as is usual, ossified with the anterior fin ray. 

 Concerning the majority of these structures nothing need be said 

 but that they are on the same plan as the rays of the unpaired fins. 

 The most anterior ray (fig. 7), however, requires special mention. It 

 is completely ossified, terminates in a sharp point, and has the 

 posterior edge serrated. By special arrangements it can be firmly 

 fixed in the erect position, and can only be depressed by rotation 

 through an angle of H0° ; it is therefore an important weapon for 

 defence or offence. These arrangements are as follows : — From the 

 upper surface of the base (the original mesopterygial basale) a high 

 semicircular ridge (sr) arises, and the proximal extremity terminates 

 in two processes (tps and tpi), including a deep groove between them. 

 When the fin is erected the semi-circular ridge runs into the semi- 

 circular groove (fig. 7 sg) at the base of the inferior process of the 

 mesoclavicula, and at the same time the outer edge of the coracoid is 

 received into the groove between the two terminal processes. Move- 

 ment directly forward or directly backward is now effectually 

 prevented, and flexion can only be accomplished by rotation, when 

 the ridge slips out of its groove, and the outer edge of the coracoid out 

 of its groove. 



