45 



and upon the superior lateral side part of another tertiary 

 series of plates. Prof. Meek called the tertiary radials in this genus 

 brachials, but we prefer to call them the tertiary radials, because 

 they rest, in part, upon the first primary radials as well as upon 

 the secondary radials and constitute part of the calyx. In some 

 rays there are three tertiary radials entering into and forming part 

 of the calyx and in other rays there are four, and these are fol- 

 lowed by the free arms. In the series resting m part on the first 

 primary radials there are usually four tertiary radials, and in the 

 series resting upon the secondary radials there are three tertiary 

 radials. The first four or five plates of the free arms are very 

 short and transverse and above these the arms are composed of a 

 double series of interlocking plates. Each plate bears a small 

 pinnule. The arms gradually taper and are depressed, convex or 

 somewhat flattened externally. There are twenty arms, four be- 

 tween interradials, and they infold at the top leaving the apex of 

 the vault uncovered and the interradials extended above them. 



The azygous plate is quadrangular, the shorter and lower lateral 

 sides rest in a notch at the angle of the pentagon formed by the 

 basal plates, while the longer convex sides truncate the inferior 

 lateral angles of the two first primary radials. The interradials, 

 or interbrachials, as they are usually called, consist of five plates 

 which divide the arms into clusters of four each. They are leaf- 

 like expansions, each of which rests in a broad, deep, angular fur- 

 row in the vault, and projects half its width beyond and above 

 the arms, where it is thin and knife-like. The thicker part of 

 each plate is within the outer margin of the infolded arms, and 

 the shape of the furrow into which the interradial is inserted may 

 be seen in figure 9, though a small piece of the interradial is 

 remaining in the lower part of the furrow. 



The vault is pyramidal and fluted or furrowed and covered with 

 large plates. In the lower part of the vault the plates are sharply 

 angular in the middle part and separated by a suture in the bot- 

 tom of the furrows, but higher up there are two angular ridges 

 on a plate, separated by a flattened space, at the bottom of the 

 furrow, without a suture. This arrangement breaks the continua- 

 tion of the sutures and of course strengthens the vault. The apex 

 of the vault is small and covered with quite small plates. A 

 Platyceras or other Gastropod could not attach to this summit on 

 account of the projecting knife-like edges of the interradials. 



