MONOGRAPH OF THE EXISTING GKINOIDS. 187 



a large dorsal ligament fossa, which is somewhat more broadly rounded distally 

 than proximally; a small but deep dorsal ligament pit cuts into the fulcral ridge 

 just beneath the central canal, which is large and transversely oval. About the 

 central canal is a broad raised area on all sides sinking gradually into the exca- 

 vation of the interarticular ligament fossae, which is continued as a narrow band 

 of diminishing height to apices at the outer ends of the fulcral ridge. Inside 

 the fulcral ridge the proximal half of the joint face is half again as high as the 

 distal, and is evenly and broadly rounded outwardly, while the latter, running in 

 a slightly convex line from the distal end of the fulcral ridge to the inner distal 

 angle, makes there a rather sharply rounded turn. A broad intermuscular 

 farrow with parallel sides runs to the run about the central canal. In the proxi- 

 mal half the interarticular ligament fossa is triangular, the distal border sloping 

 inward and downward and making an angle of about 30 with the fulcral ridge, 

 and the approximately semicircular portion of the joint face beyond is occupied 

 by a large muscular fossa of about two-thirds its area. In the distal half the 

 distal border of the interarticular ligament fossa is roughly parallel with the 

 fulcral ridge, though bending outward is a very broad curve in its inner half, 

 and the muscular fossa is more or less boomerang shaped with a short and narrow 

 inner limb coming to a point at the inner end of the intermuscular furrow, and 

 a broad slowly tapering outer limb. The muscular fossae, though well differen- 

 tiated, appear to contain very few muscle fibers. 



The articulation between the second and third segments of Pj : The articular 

 surface is semicircular outwardly, and inwardly composed of two semicircles meet- 

 ing in a deep, narrow incision in the median line. The surface is moderately 

 excavated, with a broad median fulcral ridge which runs from the incision be- 

 tween the two semicircular halves of the inner border to a point just proximal 

 to the middle of the dorsal border, including the central canal just beyond the 

 proximal third. 



The articulation between the third and fourth segments of P a : The articular 

 surface, of the same shape as that of the preceding articulation, is almost plane, 

 but the inner portion is very slightly depressed and shows a rather broad and 

 ill-defined ridge of denser tissue, appearing opaquely white against a darker back- 

 ground, which crosses the joint face from the middle of the ventral border to a 

 point considerably proximal to the middle of the dorsal border, including the 

 central canal at the end of its ventral third. 



The articulation between the distal segments of P^ : There is a broad dorso- 

 ventral fulcral ridge, and near the ventral outer corners on either side a small 

 deep approximately circular pit which lodges a small but compact and con- 

 spicuous muscle bundle. This pair of muscles continues to the pinnule tip. 



The articulation between the outer brachials and the distal pinnules: The 

 dorsal ligament fossa is deeply excavated with a deep and prominent ligament 

 pit excising the fulcral ridge and separated from the large transversely oval 

 central canal by a very narrow septum; in the distal portion of the articular 

 face there is a large, deep, undivided muscular fossa which, in the form of a 



