80 IOWA GEOLOGICAL SURVEY 



der), as it is grasped by the two articular plates of the upper 

 arm, and thus forms the brachial joint. Immediately behind 

 this brachial process is a small oval aperture, the brachial fora- 

 men, perforating the brachial fossa from the interior, and which 

 no doubt served to convey to the arm the blood-vessels and 

 nerves required for its supply. 



The ventral surface of the carapace is completed by the 

 median ventral plate (m. v., Fig 10), in the center, 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. 



Each of the hollow arms or brachia is divided by a trans- 

 verse 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 ex- 

 tremity 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 sepa- 

 rated 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 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 (•/. m.), while dorsally and ven- 



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

 plates coming together above the external marginal. 



