188 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



slightly flattened; the general appearance of the animal is rather 

 robust. 



The disk is rounded or subpentagonal. The dorsal surface is 

 covered with subequal imbricated plates, of medium size, which are 

 a little larger toward the middle of the interradial spaces, and become 

 a little smaller at the periphery of the disk ; they are quite without 

 spines. The radial shields are rather large and broad, with an 

 almost straight internal border, a strongly convex external border, 

 and a very pointed proximal angle ; the two shields of each pair are 

 close together, but not in contact distally, where they are separated by 

 a narrow plate, and they are slightly divergent proximally. They 

 are twice as long as broad, and their length is slightly less than half 

 the radius of the disk. 



The ventral surface is uniformly covered with smaller plates than 

 those of the dorsal surface which are equal and imbricated and 

 entirely without spines. The genital slits are rather broad. 



The mouth shields are of medium size, or rather small, almost as 

 long as broad, and are composed of a triangular major portion which 

 is broader than long with an obtuse and rounded proximal angle, 

 plus a distal rounded lobe which projects more or less strongly into 

 the interradial space. The acloral plates are rather small, narrow, 

 twice as long as broad, and recurved in the form of a crescent ; they 

 are in contact in the median interradial line, and outwardly they 

 approach very closely to their neighbors. The oral plates are twice 

 as high as broad. The single external mouth papilla is very large, 

 broadened, with the form of a right-angled triangle of which the 

 hypotenuse is slightly serrate and the proximal angle is more or less 

 sharp; the internal or proximal papilla is thick, with a somewhat 

 irregular free edge. 



The upper arm plates are large and greatly broadened; they are 

 relatively short in proportion to their width, and are at least two 

 and a half times as broad as long. They are quadrangular, with 

 a very small and straight proximal border, straight and vfcry 

 strongly divergent sides, and a broad straight or slightly convex 

 distal border; the lateral angles are sharp, but slightly rounded 

 at the apex. At some distance from the disk the proximal side, 

 which has become progressively reduced, disappears and is re- 

 placed by an obtuse angle. All the plates are in contact. 



The first under arm plate is small and triangular, with the apex 

 distal ; it is very narrow and is enclosed between the two neighbor- 

 ing adoral plates; sometimes these two adoral plates come into 

 contact, and when this is the case, the first under arm plate is 

 enclosed within them (fig. 2) ; sometimes also it is divided into 



