74 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



ft. 1 . The proximal borders of the muscular fossa; are parallel to the transverse 



ridge. 



f. The height of the interarticular ligament fossa 3 is much less than half the 

 distance from the middle to the ends of the transverse ridge. 



Tropiometra cncrinus. 



i'. The height of the interarticular ligament fossa; is much more than half the 

 distance from the middle to the ends of the transverse ridge. 



Tropiometra ca-rlniitn-. 

 ft*. The proximal herders of the muscular fossre run upward and outward, making 



a considerable angle with the transverse ridge. 



i 1 . The distal borders of the muscular fossae run parallel to the distal borders of 

 the interarticular ligament foss:e inward and downward from the distal 

 outer angles; large supplementary muscle plates, in addition to bosses, 



are present Tropiometra macrodiscus. 



f. The distal borders of the muscular foss;p are very broadly rounded and show 

 no relation to the distill borders of the interarticular ligament fossae; there 



are no supplementary muscle plates Tropiometra, picta. 



if. The total height of the articular foss;o is nearly or quite as great as, or much greater 



than, the length of the transverse ridge. 



c?. The outer borders of the muscular fossae are straight or slightly concave and are at 

 right angles to the transverse ridge and parallel to each other; the sides of the 

 internrticular ligament fossae are strongly convergent. 



f. The muscular fossae are short, the distance from the outer angles of the inter- 

 articular ligament fossnc to the line parallel with the transverse ridge touching 

 the most distal portion of the muscular fossa? being no greater than the width. 

 J7 1 . The Intel-muscular furrow is very broad with parallel sides: in the middle it 



bears a conspicuous and high keel Stylomctra spinifera. 



f. An intermuscular ridge instead of an intermuscular furrow is present. 



ft 1 . At the inner proximal angles of the muscular fossa- are prominent bosses and 

 also large supplement:) ry muscle plates; the outer distal angles of the mus- 

 cular fossre are well rounded and the outer borders of the interarticulnr 

 ligament fossae are concave and make a relatively large angle with the 



transverse ridge Perometra diomedcw. 



h'. No bosses or supplementary muscle plates are present; the outer distal angles 

 of the muscular fossa; are relatively sharp; the outer borders of the inter- 

 articular ligament fossae are straight and make a relatively small angle 



with the transverse ridge ixtcrnnnira macropoda. 



f. The muscular fossae are longer, the height being greater than the width. 

 j; 1 . The muscular fossa 1 are very high, the maximum height being more than three 



times the width Parametra orion. 



tf. The muscular fossae are lower, the maximum height being never but slightly 



greater than twice the width. 



ft 1 . The distal border of the muscular fossae rises from the distal outer corners up- 

 ward and inward to a rounded inner distal angle near the intermuscular 

 notch. 



V. The muscular fossae are narrow, about twice as high as wide, and are sep- 

 arated by a very broad rounded intermuscular ridge; the internrticular 

 ligament fossae are separated by a space equal to twice their width. 



Floromttra perplexa. 



i 3 . The muscular fossa' are Iiroad, much less than twice as high as wide, and are 

 separated by a very narrow intermuscular ridge; the iuterarticular 

 ligament fossae are separated by a space approximately equal to their 

 own width Florometra asperrima. 



