27 
in 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. (Pr. v. fig. 7. and 20.z.) In a specimen in my possession 
(Pt. v. fig. 8.) one part of this muscle has slid over the ridge, and its two 
branches lie united together. Atthe upper surface (Pr. v. fig. 12. and 21.) the 
lip-like prominence extends laterally in 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 (Pu. v. fig. 21. œ.) 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. (Pr. v. fig. 20. ë .) 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 rugi 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, (Pr. v. fig. 9. and 10.) 
in whose centre we notice a perforated concave tubercle, which furnishes the j 
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 intervertebral structure of fish. Sometimes a few smaller 
unperforated tubercles continue the curved direction of the lip-like process, all 
probably serving for firmer muscular adhesion, 
The Seconp CosTAL Joints (Pr. 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 conyex, 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 still 
further from the centre, thus enlarging the opening which now assumes the 
form of a funnel shaped cavity (Pu. 111. fig. 1.) The inner or truncated edge 
has below, near the margin, a lip-like process cut out in the middleof itsinfe- 
zior surface, and presenting a reflected curve, by which the joint rests on 
