DEVONIC FISHES OF THE NEW YORK FORMATIONS 2)7 



plates \a. m. d. and p. m. d^ placed one behind the other in the middle of 

 the back ; passing down also on the sides are the anterior and posterior 

 dorsolaterals {a. d. I. and p. d. /.]. On the under surface, and also taking 

 part in the formation of the sides are two pairs of plates, the anterior and 

 posterior ventrolaterals \a. v. I. and p. v. /.], of which the anterior requires 

 especial attention, as to it the pectoral limb is articulated. Near the 

 anterior extremity, of this plate, on the outer aspect and close above the 

 angle which separated the lateral from the ventral surface, is a shallow 

 excavation, from the bottom of which rises a peculiar process resembling a 

 thick walled cup or helmet, whose hollowed out " mouth " points outwards, 

 and also somewhat backwards and downwards, the cup itself being fixed by 

 a stout ridge which traverses the containing hollow from behind forwards 

 and also slightly downwards. This may be called the brachial process 

 (" helmet-process " of Pander) 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 foramen, 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 \7n. v7\ in the center, and in front by two very small semilunar plates 

 \_s. /.], each of which occupies a space cut out from the inner half of the 

 anterior margin of the antero-ventrolateral, 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 transverse elbow 

 joint into two segments, proximal and distal. The proximal segment or 

 upper arm is trigonal in transverse section, becoming more flattened towards 

 the elbow, and shows three surfaces, a dorsal slightly convex, a ventral fiat, 

 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 hollow proximal expansions grasp between them the brachial 



