78 



FOSSIL ASTEROIDEA. 



granules. Sometimes the granules are still present in situ. The pits are 

 separated in the example figured PI. XXI, fig. 1, on an average rather less 

 than their own diameter apart. There may or may not be a slight margin to the 

 plate. The centrale in a specimen R : r : : 40 : 20 measures 8'5 mm. in diameter. 

 The primary interradialia are rather smaller, being 6'7 mm. in diameter. Radially 

 the most conspicuous plates of the disc are the proximal radialia. They have a 

 very characteristic appearance, their general shape reminding one of a breastplate. 

 The remainder of the plates of the disc are of very various sizes and distributed 

 in a fairly regular manner. The general arrangement of these plates is given in 

 the general account at the conclusion of these volumes. 



The madreporite is a conspicuous plate lying at the distal end of a primary 

 interradial. The two neighbouring adradialia are notched for its reception. 



The arms are moderately produced, the major radius being about twice the 

 minor radius. Measurements of five specimens give the following : 



R r 



At the base of each arm there are five series of plates visible on the dorsal 

 surface the radialia, adradialia, and supramarginalia. All the plates at the base 

 of the arm overlap. They are of a type which may be derived from the breast- 

 plate shape mentioned above. They gradually become narrowed in length and 

 increased in breadth until they are shaped somewhat like an inverted T (PI. XX, 

 fig. 2 V). The granulation is generally confined to the central region of each plate. 



The arm about halfway along its length becomes swollen and the plates no 

 longer overlap but are contiguous. They lose their J_-shaped form, become 

 almost oblong, and at the same time rather tumid. This is especially noticeable 

 in the case of the radialia. The form of the plates is, however, rarely absolutely 

 regular, but one which is generally derivable from the breastplate shape. 



If we examine a cross-section of the arm, we see that the base of the plates 

 of the dorsal intermediate series is prolonged inwards (and ventralward), so that 

 a single isolated plate appears club-shaped. 



All the plates are pitted for granules except at the extreme margin. 



The supero-marginal plates are from twelve to thirteen in number, the infero- 

 marginals from thirteen to fourteen in number. The arm is very high and both 



