29 



The subtriangular ridge on the superior surface stands in a more slanting 

 direction, and the superior circumference on the outerside of each arm joint, 

 or that towards which the series arising from two scapulae come together, is 

 much decreased, thereby indicating that they will in the next series adhere 

 no longer. 



The FIRST FINGER JOINT (PL. vi. fig. 13. to 14.) is of a reniform shape, 

 but slightly convex on its inferior surface, and concave on the superior. On 

 the inside it is excavated, having slight lip-like processes, and behind these 

 at its lower surface, a somewhat obliquely transverse subtriangular ridge, with 

 a longitudinal groove at its apex for the admission of a similar but undivided 

 ridge of the second arm joint. On the upper surface, a simple subtriangular 

 ridge also appears, showing like that on the former, the aperture of the per- 

 forating tube; this joint is entirely free at its sides, and is generally the first 

 that sends off a tentacular process. I possess, however, a fine specimen of 

 APIOCRINITE, perhaps the finest ever found (PL. i. fig. 2.) where this took 

 place at the second arm-joint. In this individual, the tentaculum arises from 

 the arm-joint on the right-hand side of the scapula, and from its left side; 

 whereas in the case where the first finger sends it ofF, it arises from the right- 

 hand side of the joint over the left division of the scapula. The insertion 

 of the tentaculum takes place in a part of the joint which is obliquely trun- 

 cated on the right side near the superior surface, and forms a concave space 

 traversed by a ridge, having in its centre a perforated cavity. 



The SECOND FINGER JOINT (PL. vi. fig. 15. to 17.) is also nearly reniform, 

 excavated in the centre of its interior side, and truncated at both extremities, 

 thereby giving insertion on the one side for a tentaculum, and on the other 

 allowing freer scope to that arising from the first finger joint. That side which 

 lies over the insertion of the tentaculum in the former joint, is somewhat 

 thinner than the other, which formation, as it is continued alternately through 

 all the following finger joints, presents an admirable contrivance to furnish an 

 additional dimension of adhering surface, and thence firmer insertion to the, 

 tentaculum, without an undue increase in its size. On the inferior surface, 

 where the second finger joint rests on the first, and also on the superior surface, 

 it has a subtriangular ridge perforated in the centre. It also shows at its 

 superior surface, numerous radiating stria?, with interrupted ring-like mark- 

 ings of growth similar to those in the former joints. 



