﻿FROM THE UPPER GREENSAND. 



211 



(fig. 1 c) ; the base is flat ; the mouth-opening small (fig. 1 b, and the oblong vent 

 occupies a space midway between the peristome and the border. 



The ambulacral areas are formed of small irregular-shaped plates, many of a triangular 

 form. On each of these is a primary tubercle, so placed that on every three plates we 



observe an oblique disposition like this . : * The poriferous zones are very narrow, 



and there is one pair of pores opposite each of the ambulacral plates, of which five are packed 

 within the vertical depth of one inter-ambulacral. Fig. 1 e shows the structure of one of 

 these areas magnified four diameters, and the form of the individual plates is well 

 delineated in this drawing. 



The inter-ambulacral areas are four times the width of the ambulacral ; the tubercles 

 are arranged with much regularity, and two series in each area, situated about one third 

 the distance between the zones and the median sutures, are formed of larger primary 

 tubercles, which are elevated on a slight carinal ridge of the plate, and extend unin- 

 terruptedly from the apical disc to the peristome ; this ridge is faintly represented 

 by a vertical line in fig. 1 e ; besides the longitudinal series there are several other 

 tubercles very regularly arranged, and between them the surface of the plates is covered 

 with a microscopic granulation. The tubercles are larger at the base, and form hori- 

 zontal rows on the plates (fig. 1 g), where several of the basal plates are drawn four dia- 

 meters to show this arrangement of the tubercles with their encircling granules. The 

 structure of the basal portion of the single inter-ambulacrum with the oblong vent is well 

 shown in fig. 1 /, where the two primary series of tubercles flank the periprocte, and others 

 fill up the intermediate spaces ; in this figure, likewise magnified four diameters, we observe 

 that each alternate plate of the basal portion of the ambulacra supports a primary tubercle. 



The apical disc (fig. 1 d) is formed of five irregular, pear-shaped ovarial plates, four of 

 which are perforated ; the right anterolateral is larger than the others, and supports as 

 usual the madreporiform body. The five heart-shaped oculars with their microscopic 

 orbits are wedged between the oyarials ; the elements of the disc are well soldered together 

 and form a compact body. 



The mouth-opening is small, and occupies a slight central depression (fig. 1 b). 



Affinities and Differences. — This Urchin closely resembles D. cylindrica, a species 

 which presents many interesting varieties of size and form from different localities, scarcely 

 two specimens from the Grey Chalk of Folkestone, or the Lower Chalk of the South of 

 England, being precisely alike. The same remark may be made of the specimens of 

 D. cylindrica now before me from the Planer of Hanover, the Craie Chlorite of La Fauge 

 (Isere), and the Etage Cenomanien of Saint- Aignan (Drome), and of other departments of 

 France ; such being the case I accept D. Favrina only as a provisional species 

 until a series of specimens have been found which may determine more accurately 

 its specific characters. 



Locality and Stratiyraphical Position. — The specimen I have figured belongs to the 



