68 OLD RED SANDSTONE FISHES. 



The ventral surface of the carapace is completed by the median ventral plate 

 (m. v., woodcut, Fig. 35) in the centre, and in front by two very small semilunar 

 plates (s. I.), each of which occupies a space cut out from the inner half of the 

 anterior margin of the anterior ventro-lateral, and is in contact in the middle line 

 with its fellow of the opposite side. In Bothriolepis these last-mentioned plates 

 seem to be represented by a single median one. 



It must be noticed (woodcut, Fig. 35) that the ventral wall of the carapace 

 passes further forwards than the dorsal one ; it extends, indeed, so far under the 

 cranial shield that the space for the mouth between the semilunar plates behind 

 and the maxillary plates in front is extremely narrow, — so narrow that I was 

 thereby originally led into placing the two pairs of plates in apposition. In fact, 

 in more than one specimen in the Edinburgh Museum they actually do seem to be 

 in apposition. 



Each of the hollow arms or brachia is divided by a transverse elbow-joint into 

 two segments, proximal and distal. The proximal segment or upper arm is 

 trigonal in transverse section, getting more flattened towards the elbow, and 

 shows three surfaces, a dorsal slightly convex, a ventral flat, and a somewhat 

 concave internal one, the latter fitting on to the side of the carapace when the 

 arm is flexed. The proximal extremity of the arm is formed by two articular 

 plates (d. ar. and v. ar.), dorsal and ventral, whose rounded and hollowed proximal 

 expansions grasp between them the brachial cup-like process of the anterior 

 ventro-lateral 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. 1 The internal articular plate (i. ar.), placed 

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

 though it is represented in Asterolepis in PL XVIII, fig. 6 ; its free upper 

 margin is concave, forming a rounded notch, over which the nerves and nutrient 

 vessels of the arm must have passed. The external marginal (e. m.) forms the 

 whole of the outer border of the upper arm, and has nearly opposite to it the 

 smaller internal marginal {i. m,), while dorsally and ventrally this part of the limb 

 is completed by the dorsal and ventral anconeal pieces (d. a. and v. a.). 



The shape of the proximal end of the arm formed by the articular plates 

 strongly reminds us of the upper extremity of a mammalian humerus, and the 

 arrangement resembles outwardly a ball-and-socket joint. But the direction of 

 the strong ridge which supports the brachial cup, and which closely fits the slit 

 between the articular plates, necessarily restricts the movement to one of flexion 



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

 together abov e the external marginal. 



