MONOGRAPH OF THE EXISTING CKINOIDS. 75 



h'. The distal border of the musenhir fossa? is couvex with the chord of the con- 

 vexity parallel to the transverse ridge, or rises from the inner to the outer 

 distal angles. 

 i l . The outer borders of the muscular foss;e are straight; the centrodorsal bears 



a single row of cirrus sockets. 

 p. The muscular fossa? are separated interiorly by a narrow intermuscular 



ridge Cyllometra albopurpurea. 



f. The muscular fossre are separated interiorly by a broad intermuscular 



groove CyJlometra disciforiiiis. 



i*. The outer borders of the muscular fossre are more or less concave; the centro- 

 dorsal bears regular columns of cirrus sockets, at least two to a column. 

 y 1 . The muscular fossa? have the inner distal angle very broadly, and the 

 outer distal angle very narrowly, rounded, and the outer borders rather 

 strongly concave: the centrodorsal bears the cirrus sockets in 15 



crowded columns Thalassometni -rilloisa. 



j 2 . The muscular fossre have the distal border evenly convex. 



fc 1 . The interior length of the muscular fossa? is three times their width ; 

 the cirrus sockets are in 10 well spaced columns. 



Stiremetra Itreviratlia. 



k*. The interior length of the muscular fossa? is much less than three times 

 their width; the cirrus sockets are in 10 columns which are separated 



by high ridges Stenometra quinquecostata. 



e?. The outer borders of the muscular fossa; are convex, more rarely nearly straight, and 



converge more or less, usually markedly, distally. 



f. The inner sides of the interarticulnr ligament fossre are straight and run upward 

 and outward at a relatively large angle with the transverse ridge so that the 

 acute distal angle of the interarticular ligament fossa? reaches a level at or be- 

 yond the halfway point between the inner proximal and distal angles of the nar- 

 row and usually more or less sharply triangular muscular fossre. 



CHAKITOMETRIDJE. 



jT 1 . The outer borders of the interarticular ligament fossre are very strongly con- 

 cave, so that the iuterarticular ligament fossie are separated by broadly len- 

 ticular gaps interradially Crinomctru con ci nun. 



g'. The outer borders of the interarticular ligament fossre are nearly or quite 

 straight, so that the interarticular ligament fossa? are separated by narrow 

 furrows interradially. 

 h l . Muscular fossae very narrow, about three times as long interiorly as broad. 



Pifcilometra acaela. 



h'. Muscular fossre wider, about twice as long interiorly as broad. 

 i 1 . Outer border of muscular fossre longer than outer border of interarticular 



ligament fossre PachyJometra incequalix. 



i'. Outer border of interarticular ligament fossre longer than outer border of 

 muscular foss;p. 



f. Lines of growth conspicuous Charitometra basicurva. 



j'. Lines of growth inconspicuous or obsolete Perissometra angusticalyx. 



f. The distal outer angles of the interarticular ligament fossre are never so high above 

 the transverse ridge as the halfway point between the proximal ends and the 

 distal borders of the muscular fossre. 

 g*. The height of the articular faces is approximately equal to the length of the 



transverse ridges. 



h 1 . The muscular and interarticular ligament fossre are about of the same size. 

 j 1 . The outer border of the interarticular ligament and muscular fossse com- 

 bined is evenly convex ; the ridge forming the distal border of the inter- 



