﻿206 



DISCOIDEA 



impressions made by the internal ribs$ so characteristic of the latter genus. For this 

 reason I am of opinion that the history of this Urchin commences with Lamarck's description. 



The outline of Discoidea cylindrica is orbicular, the base is very flat, and the upper 

 surface convex, more or less elevated, the amount varying with its phases of growth from 

 a regularly hemispherical form in young shells, as in the specimen figured in PI. XLVI, 

 fig. 2 a, to hemispherico-cylindrical, its adult condition, as shown in PI. XLVI, fig. ] c, 

 and PI. XLVII, fig. 1 a, h. 



The inter-ambulacral areas are three times the width of the ambulacral (fig. 1 a, b, d) ; 

 the plates on the sides and lower part of the dorsal surface are broad horizontally and 

 narrow vertically (fig. 1 d), and those around the vertex are nearly square (fig. 1 e) ; 

 near the middle of each plate there is a slight elevation of the surface which, in connection 

 with others in the column of plates, form a line of carination, which extends from the 

 apical disc to the circumference, and imparts a marked feature to some old tests (PI. 

 XLVII, fig. 1). On this ridge a tubercle rather larger than the others is developed 

 (fig. 1 d) on the lower half of each plate; (this tubercle with the line of carination is indi- 

 cated in fig. 1 dhy the vertical shading). On the surface of each plate from the sides of 

 the test there are from six to seven larger, and the same number of smaller tubercles 

 (fig. Id), all of which are perforated and striated, and their areolar spaces surrounded 

 by circles of minute granules (fig. 2 d) ; the quadrate plates in the upper portion of the 

 columns support only the one tubercle growing on the line of carinations already referred 

 to (fig. 1 e). In addition to the perforated and striated tubercles, the plates are covered 

 with microscopic granules, distributed very regularly over the surface. The series of 

 plates around the angular border are narrow, and bear five or six large tubercles 

 arranged in regular horizontal rows, having areolar excavations around their base. Those 

 on the basal plates are still more conspicuously ornamented with rows of primary tuber- 

 cles, each surrounded by a depressed areola bounded by granules, which are larger and 

 more thickly set than on the upper surface. The plates round the mouth are smaller and 

 have fewer tubercles (fig. 1 b). The inferior inter-ambulacral plates bulge out on each 

 half near the margin, a prominence which is continuous with the lines of carination above, 

 and indicates the position of the internal ribs in the interior of the test. PI. XLVII, 

 fig. 2 a, b, shows the position of impressions made by the internal ribs on a well-marked 

 mould ; from this we learn that the two impressions at the base of each inter-ambulacra, 

 and the ridge at the circumference, are due in part to the internal ribs of the test. 



The ambulacral areas are narrow and lanceolate above, and of the same width and 

 more prominent at the base ; they are composed of small unequal plates, of which about 

 three correspond vertically to one inter-ambulacral plate. They are, however, very 

 irregular in size and shape, some being narrow, others rhomboidal or triangular, with 

 small wedge-shaped pieces fitted into the poriferous zones, the whole forming a kind of 

 mosaic of many-sized pieces. The plates at the base and on the upper surface are more 

 regular in form and smaller in size than those on the sides (fig. Id); each plate carries 



