77 
of their exterior integument. Thus the smooth round joint of the side arm 
shows at its external circumference an elevated rim (PL. 11. 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 the joint; the 
line of tubercles assuming a somewhat spiral turn, whereby 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 apprehend belong to this species. (PL. 11. fig. 
35. to 37.) Another portion of side arms has alsooccurred (PL. 1. 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 
is bent. 
The Pervis (Pt. 1. fig. 2. to 7.) is saucer-shaped, composed of three 
plates, forming at their superior edge five concave spaces, which are generally 
finely striated to give a firmer adhesion to the scapula. 
The Scapun# (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 witha central perforation for the reception 
of the arms. 
The Arms(Pt. 1. fig. 1. and 14. to 20.) are formed, each by a single cuneiform 
joint which adheres to the scapule. 
The Hanps. (Pu. 1. 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. 
