464 Prof. Reid on the Vogrmarus Islandicus. 



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cupied the centre of each lateral surface. The bodies of the ver- 

 tebrae were subcylindrical^ and a transverse section of the body 

 of a dorsal vertebra (fig. 3) approached an oval form, was slightly 

 longer in its vertical than in its transverse diameter, and was 

 considerably wider across the centre than at the upper and lower 

 edges, where its processes were attached. 



The last caudal vertebra was short and terminated in, or at 

 least was continuous with, a vertical plate of true cartilage, as 

 proved by microscopic examination, which was thickened along 

 its upper edge *. Along this thickened edge the caudal fins were 

 articulated. The superior spinous processes were in general very 

 slender, most of them almost thread-like, and were united 

 together by a strong dense fibrous-looking membrane (fig. 3 a) 

 continuous with the laminse or neural arches forming the 

 spinal canal. This strong fibrous membrane was prolonged up- 

 wards along the whole length of the spinous processes and also 

 enveloped the lower extremities of the interspinous or inter- 

 neural spines. These interneural spines were very flexible, ap- 

 peared to be of a cartilaginous texture, to the unaided eye, at 

 the upper part, and of a fibrous structure at the lower part. The 

 upper extremity of each became considerably expanded, chiefly 

 from before backwards ; divided itself into two limbs having a 

 notch between them ; and one of these limbs projected backwards, 

 the other forwards, the former uniting itself to the anterior limb 

 of the corresponding expansion of the interneural spine imme- 

 diately behind, the latter to the posterior limb of the correspond- 

 ing expansion of the interneural spine immediately in front ; the 

 whole together forming a band of true cartilage, as ascertained by 

 the microscope, running along the whole length of the spine, and 

 affording articular surfaces for the rays of the dorsal fin. The 

 rays of the dorsal and the interspinous or interneural spines are 

 articulated in this manner. On the upper surface of those parts of 

 this cartilaginous band formed by the junction of the two limbs 

 into which the interneural spines divide, there is a shallow arti- 

 cular excavation, elongated from before backwards. Into this 

 depression is fitted a cartilaginous disc secured by ligamentous 

 fibres in such a manner that it is permitted to move slightly in 

 all directions. The lower end of the ray of the dorsal fin bifur- 

 cates, and embracing this cartilaginous disc is firmly attached to 

 its lateral surfaces. The mode of articulation between the fin- 

 rays and the interneural spines here described, is similar to that 

 between the fin-rays and the interneural and the inferior inter- 



* This plate of cartilage ought, from its structure and relative position, 

 to be regarded as formed by interneural spines. The rays of the caudal fin 

 were placed close to each other at their articulation to the upper edge of 

 this plate, but diverged as they proceeded outwards like the rays of a fan. 



