MOKOGRAPH OF THE EXISTING CEINOIDS. 69 



but slightly flattened in the center. The two muscular fossae of each joint face 

 slope inward toward the median dorsoventral plane, so that they make with each 

 other an angle of 90°. The interarticular ligament fossae make with each other a 

 slightly greater angle: thus the central canal, which opens on a level with the 

 transverse ridge, is covered over where it passes by the muscular and interarticular 

 ligament fosste by a thin partition forming a rounded tunnel or tube, the proximal 

 end of which forms the rim about the central canal. 



The intermuscular septum is thin, but very high. Its proximal end adjoins 

 the rim about the central canal, whence it runs straight to the intermuscular notch, 

 thus gradually decreasing in height distally. 



The ventral surface of the radial pentagon is almost or quite devoid of any 

 calcareous deposit, so that the rosette is more or less visible in ventral view. 



Diameter of radial pentagon at base, 4.4 mm. ; height of radial articular face 

 (measured along the inclination), 2 mm.; width, at transverse ridge, 2.2 mm.; 

 distance from center of rosette to dorsal outer edge of radial, 2 mm. : diameter of 

 ventral interarticular sj^ace, 1.2 mm. 



PENTAMETROCRINUS SEMPEKI. 



See figs. 1006, 1007. pi. 4. 



In the following key are included all of the recent comatulids of which the 

 radial articular faces are known. 



It is understood that this key is purely tentative, for the knowledge of these 

 structures is as yet far too fragmentary to enable us to say -with any degree of 

 certainty what are and what are not the most fundamental differential characters. 

 It will be noticed that in certain cases quite unrelated species stand side by side, 

 and again closely related forms are widely separated : the probability is that when 

 these structures have been more extensively studied characters will come to light 

 by the use of which a key to the radial articular faces can be drawn up in which 

 the species will fall more in accordance with their systematic position as it is 

 known from other data. 



KET TO RECENT COMATULID.S BASED UPON CHAr.ACTERS FOfND IX ARTICULAR FACES OF RADIALS. 



a'. The radial articul.ar faces .are approxiiuatel.v crescentic .nnd witlely separated from each 

 other interradially ; the dorsal ligament fossa is nearly semicircular; the distal edge of 

 the joint face is strongly concave and parallel with the proximal border of the dorsal liga- 

 ment fossa; the outer borders of the interarticular ligament foss.-e are parallel; the mus- 

 cular fossse form curved bands of uniform width along the concave distal edges of the 



interarticular ligament foss,-? CALOMETRIDiE. 



a\ The radial articular faces are never crescentlc. 



h\ The radial articular faces resemble a very thick horseshoe, the borders of the inter- 

 articular ligament fossje, which converge distally, passing in a broad curve imper- 

 ceptibly into the proximal border of the dorsal ligament fossa, and the furrow between 

 the structures distal to the transverse ridge being very broad ; they are widely separated 

 interradially; the dorsal ligament fossa is nearly semicircular; the distal border of the 

 joint face is straight and parallel with the transverse ridge; the transverse ridge is 

 very broad and carries on either side a supplementary ligament fossa in the form of 

 a long narrow triangle, the apex of which lies near the lower border of the central 

 canal; there are no muscular fossil Pontiometra. 



