MONOGKAPH OF THE EXISTING CRINOIDS. 



41 



parallel to the outer edge of the radial face, reaching the lateral edge of the joint 

 face at a point as high above the ends of the transverse ridge as the maximum depth 

 of the dorsal ligament fossa. 



The muscular fossae are very narrow, uniform in width, strongly concave, about 

 as broad as the dorsoventral diameter of the central canal, and rim along the outer 

 edges of the inter articular ligament fossae. Interiorly their distal edges are curved 

 downward, so that their interior edges, which are parallel and run to the nm 

 about the central canal, are separated by a shallow furrow about equal to half the 

 horizontal diameter of the central canal in width ; this shallow furrow has a very 

 broadly open V- or U-shaped median notch distally, and may contain a slight 

 narrow median ridge. 



The ventral surface of the radial pentagon is hollowed out in the form of a 

 deeply concave broad pit. the edges of which are coincident with the edges of the 

 muscular fossae. A small area on the floor of this pit, just above the rosette, is 

 formed by a porous calcareous deposit. 



Diameter of radial pentagon at base, 3.7 mm. ; median height of articular face, 

 1.2 mm. ; distance from base of radial pentagon to end of interradial process, 2 mm. ; 

 width of articular face, at transverse ridge, 1.9 mm. ; distance from center of rosette 

 to middle of dorsal outer edge of radial, 1.8 mm. ; diameter of ventral surface of 

 radial pentagon, 3.2 mm. 



THALASSOMETRID.E and CHARITOMETRID^. 



The radial articular faces of the species of these two families have much in 

 common and may advantageously be discussed together. 



The interarticular ligament fossai make an angle with the dorsoventral axis 

 of the animal of about 60°, and are themselves approximately equilateral triangles 

 with their inner angles truncated. 



The muscular fossaj are usually trapezoidal or roanded triangular and are 

 parallel to the dorsoventral axis, or lie in the same planes as the interarticular liga- 

 ment fossae. 



The excavation of the various elements of the joint face is moderately de«p. 



The rim about the central canal and the distal border of the transverse ridge 

 are narrow and stand well up from the deep interarticular ligament fossae. 



The dorsal ligament fossa may be in the same plane as the interarticular liga- 

 ment fossae, or it may make a slightly lesser angle with the dorsoventral axis of 

 the animal. 



The dorsal ligament fossa is well rounded proximally and is about as high as 

 the interarticular ligament fossae. 



The centi-al canal is oval, moderate in size, or rather large. 



The muscular fossae are usually separated interiorly by an intermuscular 

 septum, which at first is high, but soon becomes low, and which is often concave or 

 grooved along its apex. In Stylornetra there is an intermuscular furrow with par- 

 allel sides, carrying in its center a rounded ridge, which is about half as wide as 

 the central canal. 



