MONOGRAPH OF THE EXISTING CEINOIDS. 



23 



border of the muscular fossae. These inner borders are parallel and are separated 

 by a narrow intermuscular furrow. The muscular fossae reach just to the distal 

 border of the central canal. 



The central part of the interior of the radial pentagon ventrally is occupied 

 by a loose calcareous deposit concealing the inner ends of the radials and the rosette 

 from ventral view. 



Diameter of radial pentagon at base, 3.3 mm. ; height of articular face (meas- 

 ured along the inclination, 1 mm.; width, at transverse ridge, 1.7 mim.; distance 

 from center of rosette to middle of dorsal outer edge of radial, 1.5 mm.; distance 

 from center of ventral face of radial pentagon to middle of ventral edge of radial, 

 0.6 mm. ; to apex of interradial angle, 1.2 mm. 



HIMEROMETEID^E and MARIAMETRIDAE. 



The Himerometridse and Mariametridae may well be consdered together, as 

 they differ but little in their structure, except in regard to the arms and pinnules. 

 I have never been able to dissect a representative of the Stephanometridae, but it 

 seems probable that the radial articular faces in this family will be found to be 

 very similar to those in the Mariametridae. 



The relation of the articular faces of the radials to the dorsoventral axis of 

 the animal varies from nearly parallel to an angle of about 60. 



The excavation or relief of the several joint face elements varies more or less 

 in different species, and is more or less inversely proportionate to the size of the 

 angle between the plane of the joint face and the dorsoventral axis, and more or 

 less directly proportionate to the size of the muscular fossae. In large species with 

 numerous arms where the muscular fossae are considerably reduced and the articular 

 faces are nearly parallel to the dorsoventral axis the excavation of the joint faces 

 as a rule is slight, as in the Comasteridse, for, as in that family, there is little 

 motion to be accommodated here ; but in smaller species with few arms, where the 

 muscular fossa? are proportionately large, the basal portion of the postradial series 

 not having become semirigid, restricting the arm movement to beyond the bases 

 of the free undivided arms, as is the case with the species with numerous arms, 

 and there is a considerable angle between the articular faces and the dorsoventral 

 axis, the sculpture of the joint faces is deep. The relief of the articular faces is, 

 in the portion distal to the transverse ridge, deeper exteriorly than interiorly. 



In large species, or in species with numerous arms, where there is but little 

 proximal interarticular motion, there is no definite raised area bordering the 

 transverse ridge anteriorly and encircling the central canal, so that the interarticu- 

 lar ligament fossae appear to extend proximally quite to the transverse ridge, as in 

 certain comasterids ; but there is always a high ridge between the central canal and 

 the deep intermuscular furrow which adjoins it distally. 



Progressive excavation begins first along the outer edge of the interarticular 

 ligament fossae and gradually extends inward and downward, so that, little by 

 little, a raised area bordering the transverse ridge anteriorly and surrounding the 

 central canal becomes evident which is at first broad, but later becomes, with 

 increasing differentiation, narrower and narrower, finally assuming the form of a 



142140 21 Bull. 82 4 



