27 



iu the centre, to admit a nourishing vessel and muscle, arising from the side 

 elevation on the central leaf of the trefoil-shaped swelling, on the inner margin 

 of the pelvis. Between these protuberances and the dividing ridge, on each 

 side extends a branch of the depressing muscle over the radiating surface to 

 the exterior margin. (PL. v. fig. 7. and 20. x.) In a specimen in my possession 

 (PL. v. fig. 8.) one part of this muscle has slid over the ridge, and its two 

 branches lie united together. At the upper surface (PL. v. fig. 12. and 21.) the 

 lip-like prominence extends laterally iu such a manner as to present the ap- 

 pearance of cordate petals, when the five joints are arranged in their natural 

 circular position. In the centre, between each two, is a very small elevation 

 inserting in a groove on the second costal joint, having behind it an oblong 

 irregular triangular ridge (Pi,, v. fig. 21. 03.) with a perforated centre, contain- 

 ing a muscle and vessel of nourishment formed by the union of those in the two 

 perforations noticed on the inferior side of this joint. (PL. v. fig. 20. $ .) The 

 short angle of the ridge extends to the lip-like process, and serves for the 

 attachment of an elevating muscle, which is to counteract the depressing one 

 noticed before ; the effects of their operations may be frequently seen in the 

 numerous rngi they formed across the truncated point. 



On each of the two lateral surfaces by which the first costals adhere to each 

 other, is a curved continuation of the lip-like process, (Pi,, v. fig. 9. and 10.) 

 in whose centre we notice a perforated concave tubercle, which furnishes the 

 idea that here, perhaps, a mechanism existed to aid motion, on the principle 

 of a ball of fluid enclosed in the concavities of two approximating tubercles ; 

 analogous to the iatervertebral structure of fish. Sometimes a few smaller 

 unperforated tubercles continue the curved direction of the lip-like process, all 

 probably berving for firmer muscular adhesion. 



The SECOND COSTAL JOINTS (PL. v. fig. 14. to 18.) bear also an affinity in 

 their general formation to the former, the larger inferior surface rests on the 

 former joints, whilst a similar superior one admits the scapula. The inferior 

 surface is convex, the superior concave, the lateral sides are bevelled, form- 

 ing spaces like those noticed when speaking of the first costal joints. 

 Their exterior circumference contracts above, and their interior withdraws 8till 

 further from the centre, thus enlarging the opening which now assumes the 

 form of a funnel shaped cavity (PL. in. fig. 1.) The inner or truncated edge 

 has below, near the margin, a lip-like process cut out in the middleof its infer 

 rior surface, and presenting a reflected curve, by which the joint rests Oft 



