MONOGRAPH OF THE EXISTING CEINOIDS. 59 



The transverse ridges of adjacent articular faces join interradially, and the 

 ends of the dorsal ligament fossae are slightly truncated and more or less con- 

 fluent. The interarticular ligament fossae are separated interradially by a narrow 

 parallel sided groove, but the thin lateral edges of the muscular fossae are in close 

 contact. 



The lateral borders of the interarticular ligament fossae make an angle of, 

 in general, about 30 with the dorsoventral axis of the animal. Their central 

 portion is straight; proximally they at first retreat rather strongly from the ends 

 of the transverse ridge, and distally they retreat, less strongly, from the lower 

 outer angles of the muscular fossae. The interarticular ligament fossae are about as 

 high as the dorsal ligament fossa ; their distal borders run from the distal angle 

 inward and downward, making an angle of between 30 and 40 with the trans- 

 verse ridge, joining the central calcareous structures at a height above the central 

 canal which is about equal to its horizontal (greater) diameter. 



The calcareous prominence about and above the central canal is large and 

 broad ; roughly, it may be described as an equilateral triangle, the base of which 

 rests upon the transverse ridge, the apex being continued anteriorly into the nar- 

 row intermuscular septum. From the outer half of each side arise the ridges 

 forming the anterior borders of the interarticular ligament fossae. These are at 

 first broad, but narrow rapidly and are slender in their outer half. The portion 

 of the central calcareous structures distal to the central canal is more or less 

 excavated, so that its edges are raised into a more or less marked rim proximally 

 and laterally. It also slopes inward at a considerable angle, so that the inter- 

 muscular septum at its inception is only about one-half the height of the ridges 

 forming the distal border of the interarticular ligament fossae. 



The intermuscular septum is comparatively low ; it decreases slowly and evenly 

 in height distally, ending at the intermuscular notch. Its sides are nearly or 

 quite parallel. 



The outer borders of the muscular fossae are parallel to the dorsoventral axis 

 of the animal, and are about as long as the outer edges of the interarticular 

 ligament fossae. The distal edges make an angle of about 40 with the transverse 

 ridge until near the median axis of the joint face, when they turn rather abruptly 

 downward to form a rather broad intermuscular notch which reaches a depth of 

 from one-third to one-half of the length of the intermuscular septum. The outer 

 distal angle of the muscular fossae, between the outer and the distal edges, like 

 the distal angle on either side of the intermuscular notch, is only moderately 

 rounded, rather less so than the latter. 



The elements of the joint face distal to the transverse ridge on either side of 

 the median line of the joint face make with each other an angle of about 90, so 

 that the inner part of the joint face is very deep; but the apex of the groove is 

 rather broadly rounded off, so that the inner part of the articular faces encroaches 

 comparatively little upon the ventral face of the radial pentagon. 



There are at most only a few threads of calcareous matter within the central 

 cavity of the radial pentagon, so that the rosette is in plain view from the 

 ventral side. 



