34 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



LAMI'ROMETRA PROTECTUS. 

 Figs. 49, 50, p. 26. 



The outer margins of the interarticular ligament fossae at first retreat rapidly, 

 but just before the middle curve around and become parallel to the dorsoventral 

 axis. The outer corners of the muscular fossa* project somewhat so that their 

 tips are on a plane parallel with the dorsoventral axis and passing through the 

 distal border of the central canal. 



The dorsal ligament fossa is narrow, about three times as broad as high, with 

 the proximal border evenly curved and turning upward abruptly at either end. 



The dorsal ligament fossa- of adjacent joint faces are entirely distinct from 

 each other, but the transverse ridges just meet. 



There is no rim about the central canal. 



In the interradial planes the interarticular ligament and muscular fossae are 

 separated by very narrow U-shaped gaps extending downward nearly to the 

 transverse ridges. 



When the interarticular ligament fossae are viewed perpendicularly their 

 lateral borders are seen to run diagonally inward at an angle of about 30 for 

 nearly half their length, and then turn upward, running parallel to the dorso- 

 ventral axis to the outer distal corners. The height of the interarticular ligament 

 fossae is about equal to the distance between the central canal and the end of 

 the transverse ridge. From the outer distal angles the distal borders of the inter- 

 articular ligament fossa; run inward in an evenly and slightly convex line, the 

 chord of the convexity making an angle of approximately 30 with the trans- 

 verse ridge, until nearly half the distance to the dorsoventral plane passing through 

 the central canal is covered, when they make a broad turn and run nearly straight, 

 at an angle of about 30 to that plane, to a point just over the lateral borders of 

 the central canal, then turn inward, meeting the edge of the other interarticular 

 ligament fossa in a straight line passing just over the distal edge of the central 

 canal. The inner distal angles, typically well rounded angles of about 120, 

 are sometimes so blunted as to be practically obsolete. The space between the 

 interarticular ligament fossae is broadly U-shaped with the sides diverging at an 

 angle of about 60, the area involved being approximately equal to the area of 

 the interarticular ligament fossa? beyond the level of the distal borders of the 

 central canal. 



The muscular fossa? are very small, their greatest diameter, just above the 

 outer distal angles of the interarticular ligament fossa 1 , being about the same as 

 the dorsoventral width of the central canal ; from this point they extend diagonally 

 outward in a semicircle or short half-ellipse ; inwardly they taper to a point which 

 lies at the distal inner angle of the interarticular ligament fossa?. 



The. ventral surface of the radial pentagon is covered with a spongy calcareous 

 mass which has an irregularly radiating surface structure and of which the sur- 

 face is plane or very slightly concave, on a level with the highest point reached by 

 the muscular foss. The central perforation is rather small. 



