248 ECHIA^OBRISSUS 



The base is concave, and the mouth-opening occupies a deep depression at the 

 junction of the anterior with the middle third of the base (fig. 3 c) ; the development of 

 the five oral lobes, and the five alternating, petaloidal rosettes impart a remarkable 

 generic character to the only specimen in which this portion of the anatomy of the test is 

 satisfactorily exposed. I have given a figure of this structure, as all the other specimens 

 have the base covered more or less with the coarse grains of the matrix. 



Affinities and Differences. — This species resembles Clypeopyrjus Cerceleti,^Oxh.,\\\ all 

 the chief points of its anatomy, but differs in the following particulars : — C. Fiifonihas 

 the test more oblong or subquadrate, and not enlarged posteiiorly ; the single inter- 

 ambulacrum is more developed, rises higher on the upper surface, and curves lower 

 on the under surface than in d'Orbigny's figure of C. CerceMi. 



Locality and Stratic/raphical Position. — This rare Urchin was collected from the 

 Lower Greensand of Shanklin, Isle of Wight, by the Rev. T. Wiltshire, F.G.S., and 

 myself. I have dedicated the species to the memory of my old friend Dr. Fitton, 

 F.R.S., whose admirable monograph on the Lower Greensand of the Isle of Wight will 

 long remain a text-book to the explorers of this classical geological region. 



Genus — Echinobrissus, Sreynius, 1732. 



KucLEOLiTES, Lamarcl', 1801. 



— Goldfuss, 1S2G. 



— Agassi:, 1837. 

 Echinobrissus, d'Orbiymj, 1855. 



— Desor, 1857. 



— Cot fern, 1858. 



— J)e Loriol, 1868. 



This natural group is composed of small Urchins which have an oval, oblong, subquadrate, 

 or subcircular form, more or less convex on the upper surface, and slightly concave at 

 the base ; the test is obtusely rounded anteriorly, more or less produced, truncated, or 

 lobed posteriorly, and in general is nari'ower at the anterior than the posterior third ; the 

 vent opens into an anal sulcus which in one group extends from the apical disc to the 

 posterior border, and in another is limited to the lower third of the inter-ambulacrum ; 

 the periprocte was closed by a series of small anal plates usually absent in fossil forms, 

 but preserved in the only living descendant of the genus. 



The base is more or less concave ; the mouth-opening is small, pentagonal, 

 excentral, and lodged in an excentral depression ; in one group the peristome forms 

 a regular pentagon ; in another group it is directed obliquely across the test. 



