77 
of their exterior integument. Thus the smooth round joint of the side arm 
shows at its external circumference an elevated rim (PL. rr. fig. 39.) the first 
vestige of contraction, which may be traced through.a series of specimens, to: 
swell and increase more and more on one side, gradually becoming tubercular, 
(PL. 11. fig. 40. to 42.) and sending off from about six to eight spine-like mus- 
cular elongations (PL. 11. fig, 43.and 44.) not unfrequently proving their former 
muscular state by ‘a curved inclination downwards. The more the muscle is. 
thus protruded, the closer it approaches to the superior surface of thejoint; the 
line of tubercles assuming a somewhat spiralturn, w hereby the joint receives a 
truncated conical figure, its superior surface forming the base of the cone. In 
young undeveloped specimens the tubercles form a still greater spiral curve. 
On the surface of the same mass of partially decomposed limestone, I have 
met with very thin joints resembling those just described, only being very 
small and much longer, which 1 apprebend belong to this species. (Pr. 11. fig. 
35. to 57.) Another portion of side arms has also occurred (Pr. II. fig. 38. and. 
45.) where the joints are elliptically compressed, articulating by finely striated 
surfaces, and having a sort of serrated projection near the upper margin of the 
joint at the exterior circumference, within the line of curvature when the arm 
15 bent. 
The Pervis (Pr. 1. fig. 2. to 7.) is saucer-shaped, composed of three 
lates, forming at their superior edge five concave spaces, which are generally 
p > > P = 2 p 2 ° Y 
finely striated to give a firmer adhesion to the scapula. 
The ScAPUI (fig. 8. to 13.) are five, tile-shaped, thin plates, adhering by 
their lateral edges to each other, truncated above, and angularly bevelled to 
these lateral edges. - In the centre, at the superior edge, they are excavated, 
having a somewhat reniform or horse-shoe shaped impression on the exterior, 
which presents a ridge of articulation with a central perforation for the reception: 
of the arms. 
The Arms(Pr. r. fig. 1. and 14. to 20.) are formed, each by a single cuneiform. 
joint which adheres to the scapulæ. 
The Hanps. (Pr. r. fig. 21. to 25.) Two hands proceed from the cuneiform 
joints of the arm, each is formed by a horse-shoe shaped joint, and a cuneiform: 
one. T 




