418 BALANIDJE. 



cliaracteristic of the present species, need not here be given. The 

 crown-like shape of the shell is well expressed by its name of Diadema, 

 but the crown tends to pass into a cylinder. The radii are extremely 

 broad. The orifice is large, and neatly hexagonal : when the oper- 

 culum is removed the whole inside of the cup-formed shell can be seen 

 at once, for the flat membranous basis is much smaller than the orifice. 

 The under side of the shell is deeply concave. The outside of each com- 

 partment is formed by broad, rounded, and somewhat prominent, rarely 

 divided, ribs (i. e. the transverse ends of the folded walls) ; these ribs 

 are closely united together by finely serrated lines of junction (PI. 16, 

 fig. 1,/). Their surfaces outside are delicately striated longitudinally, 

 and plainly crossed (more plainly than in the foregoing and the next 

 species) by irregular, transverse ridges, especially in the lower part of 

 the shell. The largest specimen which I have seen was two and a half 

 inches in diameter and two in height. 



Scuta. — These are placed close together at the rostral end of the 

 orifice, and are imbedded in the brownish, tough, longitudinally plicated, 

 horny substance, which extends far beyond both ends of the valves. In 

 outline (fig. 3 b) they are mitre-shaped, or rounded and sub-triangular, 

 a little curved, and more or less elongated, being most so in young 

 specimens ; they are, however, less elongated and rather more massive 

 than in C. balcenaris. Terga, — these seem entirely absent in most 

 specimens ; but in one (fig. 3 a) I found a rudiment, namely, a short 

 thin plate of shell, barely visible to the naked eye, extending parallel 

 and near to the tergal margin of the scutum. The lips of the aper- 

 ture of the sack are prominent, and highly so towards the carinal end. 



Structure of Shell and Radii. — Owing to the shell not spreading 

 much at the base, new folds in the walls are much seldomer formed, 

 and therefore the external longitudinal ribs (i.e. the terminal transversely 

 elongated loops), are much seldomer divided, than in C. balcenaris or 

 regince ; even rather large specimens sometimes having only the original 

 eighteen folds.* When new folds are formed, only one is formed on 

 one, viz., the rostral, side of each suture, instead of on both sides, as in 

 C. balcenaris. The inner ends of the folded walls, surrounding the 

 basal membrane, are narrow, instead of being almost square, as in C. 

 balcenaris. The lower edge of the sheath, which projects freely, 

 descends almost to the level of the basal edges of the walls. The 

 outer ends of the folded walls, forming the transverse loops, are in- 

 ternally filled up solidly by calcareous matter, instead of by septa 

 forming tubes, as in C. balcenaris. The radii (PI. 16, fig. 2) are a 

 little thicker in the lower and outer than in the upper and inner part 

 of each compartment ; in the middle, they do not reach the sheath by 



* These should be counted on the under side of the shell, for if counted from 

 the outside, the number would be thought to be twenty-four, as on the side of 

 each compartment bordering the radius, a half-fold has the appearance of 

 being a whole fold, so that in appearance six folds are added to the real number 

 of the folds. This caution is necessary whatever the number of real folds may 

 be, that is, whether or not the original eighteen folds have been increased by 

 subsequent plications. These remarks, also, are applicable to the other species. 



