77 



of their exterior integument. Thus the smooth round joint of the side arm 

 shows at its external circumference an elevated rim (Pi,, n. 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, 

 (?L. ii. fig. 40. to 42.) and sending off from about six to eight spine-like mus- 

 cular elongations (PL. ii. fig. 43. and 44) not [infrequently 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. la 

 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. ii. fig. 

 35. to 37.) Another portion of side arms has also occurred (PL. n. 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 PELVIS (PL. i. 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 scapulae. 



The SCAPUIJE (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(PL. i. fig. 1 . and 14. to 20.) are formed, each by a single cuneiform 

 joint which adheres to the scapulae. 



The HANDS. (PL. i. 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. 



