300 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



and unites with the horizontal plate forming the surface on which 

 the lateral processes of its ray rests. 



The succeeding rays and their interspinalia are not modified. The 

 latter, five in number, lie below in the cleft extremities of the spinous 

 processes of the 6th-l 0th vertebrae. The rays are slightly expanded 

 and osseous below, but towards the extremities are horny, trans- 

 versely striated, and branch dichotomously. 



A study of the development of these bones throws light on their 

 homologies. The horizontal plate which supports the defensive ray, 

 and the anterior prolongation of it, are formed in membrane (Fig. 10, 

 hp). The small ossicle lying in front of it is represented at an early 

 stage by a rod of cartilage, (Isp.l), lying almost in the longitudinal 

 axis of the body. The small n -shaped bone is also developed in 

 membrane, the bone on which it rests being partly formed in cartil- 

 age, (Isp.2) and partly (i.e., the posterior part) in membrane. The 

 defensive ray and its successors are formed in membrane, and its 

 interspinal (lsp.3) and its successors are preformed in cartilage. These, 

 then, being the facts, one must refer all those bones which are pre- 

 formed in cartilage to the category of interspinals, and all those 

 formed in membrane to that of rays. Accordingly, the anterior 

 bone, which is united by ligament to the horizontal supporting plate, 

 is the first interspinal, which early (even while completely cartilagin- 

 ous) has lost its typical position, and the horizontal supporting plate, 

 the anterior portion of it at any rate, is to be considered the ray 

 corresponding to it. The interspinal enclosed within the strong 

 fourth spinous process is then the 2nd, the small ossicle which it 

 supports being the 2nd ray. This second interspinal has a certain 

 amount of membrane united to it ; the lateral flanges which give a 

 point d'appui for the limbs of the 2nd ray, the thin plate uniting it 

 with the 3rd interspinal, and the portion of its extremity behind the 

 slight groove (in reality a continuation of the thin plate), being of 

 this nature. The third interspinal is also formed partly of cartilage 

 and partly of membrane-bone, the portion of the horizontal plate in 

 which the 3rd ray rests probably belonging to the membranous por- 

 tion of the 3rd interspinal, which has coalesced with the modified 1st 

 ray. 



