38 NEW YORK STATE MUSEUM 



cuplike process of the antero-ventrolateral plate of the body. These plates 

 are consequently not in apposition at the joint, but are separated by an 

 interval or slit, which contains and moves on the ridge attaching the brachial 

 cup to the bottom of its fossa ; and this interval is closed internally by the 

 internal articular plate, and externally by the upper narrow extremity of the 

 external marginal/ The internal articular plate [/. ar.\ placed right on 

 the inner surface of the arm below the joint, is not seen in these figures; 

 its free upper margin is concave, forming a rounded notch, over which the 

 nerves and nutrient vessels of the appendage must have passed. The 

 external marginal [c. m.~\ forms the whole of the outer border of the upper 

 arm, and has nearly opposite to it the smaller internal marginal \J. w.], while 

 dorsally and ventrally this part of the limb is completed by the dorsal and 

 ventral anconeal pieces [d. a. and v. «.]. 



The distal portion of the limb, or lower arm, is more flattened, and 

 shows a dorsal and ventral surface, two sharp margins, external and internal, 

 and a sharp apex or point. The ventral surface, and presumably the dorsal 

 as well, is composed of two central pieces [c], and six marginals [;«] three of 

 which are external, and three internal. It is the distal marginal on the 

 inner side which forms the acute point or apex of the appendage, as first 

 noticed by Jaekel. 



The elbow joint is somewhat complicated. Each upper marginal of 

 the lower arm is furnished above with an articular process, which is received 

 within the lower extremities of the internal and external marginal plates 

 respectively of the upper part of the limb. Then, on the other hand, each 

 anconeal plate, dorsal and ventral, of the upper arm has a small flat articular 

 process below, which fits into a slit on the outer surface of the upper 

 extremity of the corresponding upper central of the forearm. It is hard to 

 say how much movement could have been here allowed, but from the form 

 of the joint it seems probable it was limited to a slight flexion and extension, 

 and possibly only in the horizontal plane, as in the case of the shoulder. 



■ In Bothriolepis, however, the slit is completed externally by the two articular plates 

 coming together above the external marginal. 



