FROM THE UPPER GREENSAND. 209 



Description. — The specimens of this Urchin sent to me for drawing and description 

 were so imperfect that I requested Mr. Bone to make accurate copies of the very capital 

 figures of this species given by my friend M. Cotteau in the ' Paleontologie Fran9aise,* 

 pi. 1012, figs. 1—7. 



The test is small and sub-circular ; the upper surface inflated and convex ; the 

 base slightly swollen in the middle, and round at the border. 



The ambulacra half the width of the inter-ambulacra, with two rows of primary 

 tubercles extending from the disc to the peristome (fig. 4 r/, h) ; the plates supporting, 

 besides, numerous fine granules, which are disposed around the base of the tubercles. 

 The poriferous zones are narrow, the pores round, unigeminal, slightly oblique, and well 

 spaced out from each other (fig. 4/ and fig. 4 ff). 



The inter-ambulacra are formed of large plates ; at the ambitus each plate carries 

 three tubercles, two of which are more conspicuous and persistent than the others, and 

 reach higher up the sides. The central tubercles extend from the disc to the peristome, 

 and the zonal series is absent above and below ; the larger tubercles grow on two slightly 

 elevated longitudinal carinal lines (fig. 4/), indicated by the direction of the shading in 

 the figures. The granulations on the plates are very fine and form horizontal beaded 

 chains of great delicacy ; the primary tubercles at their base have circular depressed 

 areolas without encircling granulations. 



The small mouth-opening has a circular or sub-decagonal peristome and opens in the 

 centre of the base, which is shghtly convex (fig. 4 a). 



The vent is oval, and acuminated at the inner extremity ; it occupies nearly two 

 thirds of the space between the peristome and posterior border ; the aperture was closed 

 by a series of ten anal plates, unequal in size (fig. 4 h) and covered with small granules ; 

 the periprocte at the inner border is surrounded by seven small plates let into a space 

 around the vent like a piece of mosaic. 



The apical disc is small and composed of five perforated ovarial plates, the right 

 antero-lateral being much the largest and covered with the madreporiform body ; the 

 surface of the other four is crowded with granules ; and the small cordate oculars are 

 wedged into the angles formed by the ovarial plates. 



Jffinities and Differences. — The general form and arrangement of the tubercles on 

 D. minima resemble the young condition of D. subucuhts, of which some authors 

 consider it a small variety. M. Cotteau^ has had an opportunity of examining with care a 

 great number of specimens collected from different localities, and says that he has 

 acquired the certainty that this species differs essentially from D. subuculus, not only in 

 its size, which is constantly smaller, but in its greater height, more inflated sides, and 

 convex base, and by its granules, which ai'e more closely set together and disposed in 

 more regular lines on the plates, and always by its apical disc, which is composed of five 

 perforated ovarial plates. 



1 ' Paleontologie Fran9ai8e,' tome vii, p. 35. 



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