42 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The muscular fossae are always higher than the interarticular ligament fossae, 

 sometimes much higher, and they may include one or two pairs of supplementary 

 fossae, or may show two or three ridges parallel to their distal borders. There is 

 an intermuscular notch, which may be broad and shallow, or deep and very narrow. 



There is a loose deposit of calcareous matter on the inner surface of the radials, 

 but it rarely conceals them entirely, or obscures the central canal. 



The ventral surface of the radial pentagon is stellate in outline, but the incisions 

 are rarely deep. 



THALASSOMETEID^. 



The muscular fossae lie in a plane nearly or quite parallel to the dorsoventral 

 axis of the animal, which makes an angle of nearly or quite 120 with the plane 

 of the interarticular ligament fossae. They are more or less trapezoidal in shape. 

 Their area is almost always somewhat, usually much, greater than that of the inter- 

 articular ligament fossae. 



The distal (inner) borders of the interarticular ligament fossae make an angle 

 of 60 or rather less with the transverse ridge. 



In the immature stages the radial articular faces of the species of Thalas- 

 sometridae appear to resemble those of the species of Charitometridse, suggesting, as 

 do the cirri, that the Thalassometridae are more specialized than the Charitometridse. 



PTILOMETRIN.E. 



PTILOMETRA MULLERI. 

 Figs. 67, 68, p. 43. 



The radial articular faces are, in reference to the dorsoventral axis of the 

 animal, inclined at an angle of about 45, so that they are about at right angles to 

 each other. 



The portion distal to the transverse ridge is rectangular, about three times as 

 broad as long. Interradially the dorsal ligament fossae are entirely separated, but 

 the ends of the transverse ridges and the elements of the joint faces distal to them 

 are in contact with the same parts of the adjacent joint faces. 



The plane of the dorsal ligament fossa makes a very broadly obtuse angle with 

 the plane of the other elements of the joint face. 



The dorsal ligament fossa is usually not quite twice as broad as long; the 

 proximal border is evenly curved. 



The ridge about the central canal is broad and prominent, laterally falling 

 away, sometimes almost in a straight line, to the ends of the transverse ridge. 



The interarticular ligament fossae are nearly or quite triangular, the distal 

 edges being nearly or quite horizontal, and the greatest (horizontal) length being 

 about twice the maximum vertical height ; the outer borders are usually somewhat 

 concave ; the inner apices rest on the distal side of the dorsal part of the rim about 

 the central canal, and are separated by a gap equal in breadth to about half the 

 horizontal diameter of the central canal. 



