' 26 
` joint last described. They adhere by their lateral surfaces together, and the 
whole row forms thus a kind of basin, having a subpentangular aperture in the 
centre, arising from the truncated terminations of their inner edges. The 
superior surface of each has a ridge in the centre sloping towards the lateral 
surfaces, and thus, between every contiguous pair of joints, a depression is 
formed for the insertion of the first costal joints. 
There generally exists at the lower surface (PL. v. fig. 2.) a small slightly- 
arched space, between the lateral surfaces, arising from their edges being 
bevelled, which was occupied by an intervening muscular organization, and may 
have facilitated a gradual sliding outwards of these joints, when the increase 
‘of the central aperture became necessary to the growth of the animal; and 
tended also to preserve a more free and easy motion of these parts. The ex- 
terior circumference of each plate is contracted towards the angles, and swells 
slightly to the middle. These joints become gradually thinner and narrower 
towards the truncated points, each of them having moderately thick trefoil- 
shaped elevations. The central leaf of this trefoil has a small tubercle on each 
side (Pr. v. fig. 19 2.) which serves by its insertion into a small concavity of the 
first contiguous costal joint above, to aid its motion (PL. v. fig. 20 3 .) to which 
also a nourishing vessel extends (Pr. v. fig.20 2 د‎ from a perforation in each of 
the side leaves of the trefoil elevation on the pelvis. (PL. v. fig. 19 3). 
The First Costat Joints (Pr. v. fig. 7. to 12. 20. and 21.) nearly resemble 
those of the pelvis, and insert into the subconcave or subtriangular cavities 
formed by them, yet so as to leave in the interior an open space. They have 
also, like the former plates, an arched groove between their lateral sur- 
faces, resulting from their bevelled edges. 
Their inferior surface has an elevated ridge, their superior is concave. 
Their exterior circumference is smallest at the upper part, and they do not 
extend so far in the interior as the plates of the pelvis. The truncated point is 
somewhat excavated (Pr. v. fig. 21.) and slopes off gradually towards the 
upper and under surface, having beneath a lip-like process (Pr. v. fig. 20.) over 
which the depressing muscle slides, which is affixed to two side elevations 
of the two contiguous joints of the pelvis, and divides as it proceeds (Pr. v. 
fig. 15. and 16.) leaving a swelling in the centre to attach to a lip-like promin- 
ence on the upper margin. At the lower surface, behind the lip-like process, 
on each side of the dividing ridge, is a protuberance (Pr. v. fig.20 2) perforated 
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