28 



the lip-like ridge of the former. Behind this, is again as on the superior sur- 

 face of the first costal joint, a transverse sub triangular ridge (PL. T. fig. 22. 02.) 

 with its perforation, to which the depressing muscle of the lip of the second 

 costal joint is attached. Near the margin of the superior surface is an elevated 

 rim, having rounded corners, and an angular depression in the centre, behind 

 which is a perforated tubercle, and a muscle analogous to that on the lower 

 surface of the first costals. 



The SCAPULA: (Pi,, vi. fig. 1. to 5.) contract so much at their upper circum- 

 ference, as to complete, in conjunction with the two following joints, the pyri- 

 form shape of the superior part of this species of Apiocrinites. At the inferior 

 surface they are convex, and have at the truncated or inner end a perforated 

 tubercle behind the margin, with a muscular elongation, similar to that on the 

 superior part of the second costal joint. The superior surface has an elevated 

 ridge, forming on either side two concavities for the insertion of the two first arm 

 joints. At the truncated end a lip-like process exists, behind which is situated 

 a trefoil-shaped elevation of a more extended, but somewhat similar, figure to 

 that before noticed in the pelvis, with two perforated lateral tubercles. 



With the scapulae the division of the circular area of this part of the animal 

 into five joints ceases, and another of ten commences, which gives rise to that 

 number of arms and fingers. 



The FIRST ARM JOINT (PL. vi. fig. 6. to 7.) resembles in every particular 

 the second costal joint, only that it is considerably smaller and thicker in pro- 

 portion, and that the perforated tubercle stands somewhat more towards the 

 side, which comes in contact with the fellow arm joint belonging to the same 

 scapula. This evidently results from an impeded increase, arising from want 

 of room for extension in that direction, and from the lateral power exerted by 

 the muscle proceeding through the perforation from the lower surface of the 

 scapula, where it begins to divide. 



The SECOND ARM JOINT. (PL. vi. fig. 9. to 11.) As the first arm joint 

 was observed to resemble the second costal joint, so the second arm joint will 

 be found somewhat analogous to the first costal; excepting that it wants the 

 lower central ridge, that it is thicker, and like the joint on which it rests, abbre- 

 viated on the inner side, next to its fellow in the series arising from the same 

 scapula. 



