﻿FROM THE LOWER CHALK. 



263 



these nodules present is admirably shown in three inter-ambulacral plates taken from the 

 ambitus and magnified three diameters. 



The peripetalous fasciole is very well defined in this species ; it passes straight from 

 point to point with scarcely any curvature, and forms a bold line among the numerous 

 tubercles ; fig. 1 g shows this structure. 



The tubercles are small, very numerous, and set irregularly on the plates ; they are 

 all perforated, and raised on bosses surrounded by well-defined areolas ; fig. 1 i shows 

 the tubercles on the upper surface, and fig. 1 k those on the under surface, where the 

 larger tubercles have the bosses crenulated ; besides the tubercles the entire surface of the 

 plates is closely covered with miliary granules. 



The apical disc is small and excentral, the four genital plates are perforated, and the 

 anterolateral carries the spongy body (fig. 1 /) ; the five ocular plates are very small, as 

 shown in fig. 1 /. 



The oval periprocte occupies the upper third of the oblique posterior border (fig. 1 d), 

 and the vertex is seen rising above it all, as shown in fig. 1 c, d). 



The base is convex transversely behind the mouth (fig. 1 b, c), and flat before that 

 aperture. The basal portions of the ambulacral areas, especially the postero-lateral pair, 

 which first descend backwards towards the ambitus, bend round the border forming an 

 obtuse angle there, and make a sinuous course to the mouth. The anterior pair and the 

 single area have a more direct course ; the basal portions of the postero-laterals are 

 destitute of tubercles and granules (fig. 1 b). 



The mouth-opening is situated at the anterior fourth of the base ; it is transversely 

 arched and bilabiate, the lower lip being the most prominent, and the peristome is 

 surrounded by a narrow calcareous band. 



The tubercles on the basal portion of the inter-ambulacrum have a remarkable 

 arrangement. They form a series of curved rows that radiate from a central nodule 

 near the posterior border and from a kind of fan-shaped tubercular sculpture between the 

 two smooth winding paths formed by the sinuous ambulacra (fig. b). The tubercles on 

 the other portions of the inter-ambulacra have a much less regular arrangement. 



Affinities and Differences. — This species resembles II. prunella, Desor, with which it 

 has been confounded ; it differs from that species, however, in being much larger, less 

 globular and inflated, having the posterior border obliquely truncated, the dorsum much 

 more inclined, and having the tubercles smaller and more numerous, and the inter- 

 ambulacra nodulated around the sides. 



Locality and Stratigraphical Position. — This species is found only in the Grey 

 Chalk near Folkestone, the Lower Chalk at Hamsey, Sussex, and in the Grey Chalk of 

 Ventnor, Isle of Wight. 



