MONOGRAPH OF THE EXISTING CRTNOIDS. 



There is a more or less abrupt change in direction between the outer edge 

 of the interarticular ligament fossse and that of the muscular fossae, the latter 

 projecting more or less outward. 



The muscular fossa; do not extend inward beyond the inner distal angle of 

 the interarticular ligament fossae. 



The muscular fossae slope more or less downward from the center of the 

 joint face to the periphery. 



In addition to these characters tlie centrodorsal usually bears more than one 

 row of cirrus sockets. If only one row is present the sockets composing it are 

 more or less isolated and irregular in position. 



COMATELLA NIGRA. 

 Fig.s. : , 2, p. 6. 



The dorsal ligament fossa is narrow, nearly or quite live times as broad 

 as high, the proximal edge mostly parallel to the transverse ridge but curving 

 upward somewhat strongly at either end. 



The articular faces are separated in the interradial angles by a moderate, 

 slightly rhombic, groove. The transverse ridges of adjacent faces are entirely 

 separated. 



The lateral edges of the interarticular ligament fossas retreat rapidly from 

 the ends of the transverse ridge, with which they make an angle of approxi- 

 mately 45° ; just below the muscular fossaj they turn upward and outward rather 

 sharply. The distal edge of the interarticular ligament fossse rises inward from 

 the outer distal corners until a height is reached which is about half again as 

 far from the transverse ridge perpendicularly as is the distal outer corner. From 

 this point the anterior edge makes a broad curve inward and downward, pass- 

 ing over into the parallel inner edges of the interarticular ligament fossce. These 

 parallel inner edges are separated by a broad shallow furrow, which may have a 

 low, rounded, median convexity indicated, which is about as broad as the trans- 

 verse diameter of the strongly oval central canal, from wliich it is separated 

 proximally by a narrow ill-defined rim. 



The muscular fossa? are small, rounded triangular in shape; their lateral 

 edge makes a right angle with the lateral edges of the interarticular ligament 

 fossse, but soon curves upward and inward rather sharply, then running inward 

 almost horizontally or slightly downward, almo.st straight or slightly convex, 

 to a point slightly over one-third of the way across the distal edge of the articular 

 face, where it becomes suddenly decurved to the distal border of the interarticular 

 ligament fossa, terminating the muscular fossse in a rounded apex interiorly. 



The center of the radial pentagon is occupied ventrally by a scanty loose cal- 

 careous deposit which barely conceals the rosette from ventral view; the ventral 

 sutures between the radials are prominent. 



Diameter of radial pentagon at base, 8 mm. ; height of articular face, 3.3 mm. ; 

 width, at transverse ridge, 6 mm. ; distance from center of rosette to middle of dorsal 



