46 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The muscular fossae are very long and narrow. The sides at first converge, 

 as the outer edges continue the direction of the lateral edges of the interarticular 

 ligament fossae, but soon become parallel, and near the tip diverge very slightly. The 

 length of the muscular fossae is somewhat over twice the length of their proximal 

 border, which latter is nearly twice their distal width. The two muscular fossae 

 are separated internally by a narrow rounded ridge arising from the median distal 

 portion of the rim about the central canal, at first as high as that rim, but gradually 

 decreasing in height and terminally, as the roimded distal ends of the muscular 

 fossae begin to separate, branching, and dying away in a slight thickening of the 

 inner side of the distal edge. 



There is no ventral calcareous deposit between the radials. 



Diameter of radial pentagon at base, 3.9 mm. ; height of articular face (meas- 

 ured along the inclination), 2.5 mm.; width, at transverse ridge, 2 mm.; distance 

 from center of rosette to middle of dorsal outer edge of radial, 1.6 mm. ; diameter of 

 ventral interarticular space, 1 mm. 



THALA.SSOMETRA AOASSIZII. 

 See figs. 981-983, pi. 3. 



STYLOMETRA SPINrFERA. 

 Figs. 72, 73, p. 43. 



The joint face elements distal to the transverse ridge slant inward at an angle 

 of nearly 45° with the dorsoventral axis of the animal, so that they make almost 

 a right angle with each other; the plane of the edges of the dorsal ligament fossa 

 makes a very broadly obtuse angle with the remaining elements of the joint face; 

 the sides of the interarticular ligament and muscular fossae together are slightly 

 and evenly concave. The rim about the central canal, which is large and more 

 nearly circular than usual, is prominent but low. 



Adjacent radial articular faces are separated slightly over the interradial 

 planes by very narrowly V-shaped gaps which extend downward nearly to the 

 transverse ridges. 



The dorsal ligament fossa is narrow, nearly or quite three times as broad as 

 long, moderately curved proximally, but becoming more nearly straight toward 

 either side ; the ends are slightly truncated, so that the ends of the dorsal ligament 

 bundles, as well as the ends of the transverse ridges, are in contact interradially. 



The outer sides of the interarticular ligament fossae are somewhat concave; 

 the general shape of the interarticular ligament fossae is that of an equilateral 

 triangle of which the inner angles woidd he, if not broadly truncated, at the center 

 of the proximal border of the central canal; the distal angle is rendered especially 

 acute by the concavity of the outer side. 



The muscular fossae are comparatively small, their area being approximately 

 the same as that of the interarticular ligament fossfe. Their shape suggests that 

 of the shells of such genera as Pinna; their inner borders are straight and parallel. 



