﻿ON THE ECHINOIDEA. 



457 



The spines are long and slender, the head is stout, the margin of the acetabulum 

 deeply crenulated (fig. 3 b) ; the milled ring prominent, the neck short, and both are 

 sculptured with fine, longitudinal lines ; the long, slender stem is nearly of the same 

 thickness throughout, tapering to a point near the extremity. The proportionate length 

 of the spine to the diameter of the test cannot be ascertained, for, although entire 

 individual spines are abundant in the marl, those attached to the test are nearly all 

 fractured. 



Affinities and differences. — As P. lobatum is the oldest representative of the genus 

 Pseudodiadema, in the Oolitic rocks, it is unfortunate, from the crushed state in which 

 the test is found, that a critical comparison cannot be made between this species and 

 its other Oolitic congeners. The very narrow ambulacra have few tubercles thereon, but 

 as all the tests I have examined are fractured across this part, the details of its structure 

 cannot be seen. The affinities between this urchin and some of the depressed Acrosalenias 

 is considerable, and the length of the spines in proportion to the size of the test renders 

 that relation still more remarkable. 



Locality and Stratigraphical position. — This urchin was recently discovered at Pinhay 

 Bay, near Lyme Regis, in a bed of mottled clay, on the shore at low-water mark ; 

 many tests were found together, with numerous long, slender spines strewed in abundance 

 amongst them ; some of the spines were attached to their respective tubercles, so that the 

 identity of the spines is satisfactorily proved. This bed of clay appertains to the lower 

 division of the Lower Lias, and may probably correspond to a similar urchin vein found 

 in Warwickshire and Gloucestershire, at the base of the zone of Ammonites planorbis. 



Hemipedina Tomesii, Wright, nov. sp. 

 Hemipedina Tomesii, Wr. 



Hemipedina Tomesii, Wr. 



