﻿216 



ECHINOCONUS 



Diagnosis. — Test oval, pentagonal, enlarged anteriorly, contracted and rounded- 

 posteriorly ; upper surface elevated and convex ; sides inflated ; base contracted, slightly 

 convex or flat. Mouth-opening small, roundish, and central ; vent large, elliptical,, 

 marginal ; ambulacra narrow, prominent, corresponding to the angles of the test. 

 Poriferous zones linear, pores unigeminal ; apical disc small, central, soldered to the plates 

 of the test. 



Dimensions. — The following measurement of six specimens is given to show the 

 variability of the proportions of this species. 





A. 



B. 



C. 



D. 



E. 



F. 





G. C. 



G. C. 



G. C. 



U. G. 



C. M. 



C. M. 



Length 



2 



l-6_ 

 1 10 





1^0 



1 



9 



1 



Breadth 



1-8- 

 1 1 



1 io 



llV 



1 



9 



10 



9 



1 



Height 



1 1 



1 20 



1 7 

 20 



9 



10 



7 



1 



6 



1 



G. c. means Gritty Chalk ; u. G. Upper Greensand ; c. M. Chloritic Marl ; and the measurements are 

 in inches and parts of an inch. 



Description. — The two Urchins, from the cretaceous rocks of Savoy, drawn in pi. q,. 

 figs. 14 and 17, of M. Alex. Brongniart's 'Description Geologique des Environs de 

 Paris,' and described under the names Nucleolites castanea, Al. Br., fig. 14, and 

 Nucleolites depressa, Al. Br., fig. 17, have occasioned much diversity of opinion among 

 Palaeontologists in consequence of the unsatisfactory condition of the specimens them- 

 selves and the inadequacy of the figures for subsequent determination. 



Having collected many specimens of this species showing its various stages of growth, 

 I am enabled to state that the two forms figured by Brongniart are only different conditions 

 of the same species, my larger shells agreeing with N. castanea and the smaller with 

 N. depressa. 



I have given the anatomy of the test with ample details of structure of the largest 

 specimen I have seen from a very fine fossil in my collection (fig. 2 a — g) ; the smaller 

 and commoner shell is drawn in fig. 3. 



The distinctness of the two forms has been persistently maintained by most foreign 

 authors, as our table of synonyms shows ; nevertheless in this, as in other matters, 

 authority must bend to facts, and dry anatomy settle the question of the unity of the 

 species. 



The British specimens of UcMnoconus castanea nearly all belong to the small variety ; 

 these were collected from the Chloritic Marl near Chaldon, Dorset, and most of them 

 have the test beautifully preserved. My larger specimens were obtained from the bed of 

 hard gritty siliceous Chalk near Folkestone, and are equally well preserved, so that both 

 varieties belong to the lower portion of the Cretaceous formation. 



All my specimens are of an ovate or sub-globular form, inclining to a pentangular or 



