74 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



1^. The proximal borders of the muscuhir fossfe are parallel to the transverse 

 ridge. 

 i'. The height of the interarticular ligament fossae is much less than half the 

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



Tropiometra encrinus. 

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

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



Tropiometra carinata. 



h'. The proximal borders of the muscular fossre run upward and outward, making 



a considerable angle with the transverse ridge. 



t*. The distal borders of the muscular fossae run parallel to the distal borders of 



the interarticular ligament fossae inward and downward from the distal 



outer angles ; large supplementarj' muscle plates, in addition to bosses, 



are present Tropiometra macrodiscus. 



i". The distal borders of the muscular fossa> are very broadly rounded and show 

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



are no supplementary muscle plates Tropiometra picta. 



cf. The total height of the articular fossae is nearly or quite as great as, or much greater 

 than, the length of the transverse ridge, 

 e". The outer borders of the muscular fossae are straight or slightl.y concave and are at 

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

 interarticular ligament fossae are strongly convergent. 

 f. The muscular fossje are short, the dist<ince from the outer angles of the inter- 

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

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

 g^. The intermuscular furrow is very broad with parallel sides: in the middle it 



bears a conspicuous and high keel Stylometra spinifera. 



g'. An intermuscular ridge instead of an intermuscular furrow is present. 



/^^ At the inner proximal angles of the muscular fossae are prominent bosses and 

 also large supplementary muscle plates; the outer distal angles of the mus- 

 cular fossae are well rounded and the outer borders of the interarticular 

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



transverse ridge Perometra diomedew. 



ft". No bosses or supplementary muscle plates are present; the outer distal angles 

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

 articular ligament fossae are straight and m.ike a relatively small angle 



with the transverse ridge Asterometra macropoda. 



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

 ff*. The muscular fo.ssa? are very high, the maximum height being more than three 



times the width Parametra orion. 



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

 greater than twice the width. 

 7i'. 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. 

 »'. The muscular fossje are narrow, about twice as high as wide, and are sep- 

 arated by a very broad rounded intermuscular ridge; the interarticular 

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



Florometra perplexa. 



f. The muscular foss.ne are broad, much less than twice as high as wide, and are 



separated by a very narrow intermuscular ridge; the interarticular 



ligament fossae are separated by a space approximately equal to their 



own width Florometra asperrima. 



