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 (Pi>. 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 (PL. v. fig. 19j?.) which serves by its insertion into a small concavity of the 

 first contiguous costal joint above, to aid its motion (PL. v. fig. 201..) to which 

 also a nourishing vessel extends (Pi,, v. fig. 20 <L.) from a perforation in each of 

 the side leaves of the trefoil elevation on the pelvis. (PL. v. fig. 19 j). 



The FIRST COSTAL JOINTS (Pi-, 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 ( PL. v. fig. 21.) and slopes off gradually towards the 

 upper and under surface, having beneath a lip-like process (PL. 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 (?L. v. 

 fig. 15. and 16.) leaving a swelling in the centre to attach to a lip-fike 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 (PL. v. fig. 20 <L) perforated 



