FROM THE INFERIOR OOLITE. 365 



the four ovarial holes project into the inter-ambnlacral spaces, and the five ocular holes are 

 seen at the apices of the ambulacra (PI. XXIX, fig. 1 d). The madrcporiform body 

 occupies the centre of the disc, and presents a fine spongy structure on its surface. 



The base is flat, or slightly concave ; the basal portions of the inter-anibulacral areas 

 are convex, and separated from each other by narrow, depressed ambulacral valleys 

 (PI. XXVIII, fig. 1 h), which radiate in straight lines from the mouth to the border, the 

 margin of the ambulacra being defined by the narrow poriferous zones. 



The mouth is excentral, and situated nearer the anterior than the posterior border 

 (PI. XXVIII, fig. 1 I). The peristome is surrounded by five prominent oral lobes, 

 which are separated from each other by the depressed ambulacra, as they radiate from 

 the mouth. At PI. XXIX, fig. 1 h, another view is given of this-quinque-lobed opening. 



There is a very marked distinction in the varieties of Clijpeus Plotii collected from the 

 Inferior Oolite and the Great Oolite ; so much so, that they have been considered, by good 

 local observers, to belong to two distinct species. On this point my excellent friend, the 

 Rev. A. W: Griesbach, remarks — 



" Clypeus Plotii. — I have two urchins on the table, both said to be this species. One 

 of them is a very good specimen from the Great Oolite, Kingsthorp, the other an Inferior 

 Oolite specimen from Rodborough Hill. These urchins appear to belong to two distinct 

 species, for I think their differences can hardly be less than specific. I have no doubt but 

 you have specimens both from the Great and Inferior Oolite. Pray compare them. I 

 will just say, meanwhile, that in the Inferior Oolite specimen the apical disc is all hut 

 central, while in the Great Oolite one it is much nearer the posterior margin. In the 

 Great Oolite specimen the apices of the ambulacra are deeply sunk beloio the pjlane of the 

 test, while in the Inferior Oolite specimen they are in the same plane as the test. And, in 

 the Inferior Oolite specimen, the region of the shell which contains the anal furrow is very 

 tumid, and towards the disc nearly as hiyh as the highest part of the shell ; in the Great 

 Oolite specimen the posterior part of the test corresponding is extremely and ahriqjtly 

 depressed and flattened. In general form the Inferior Oolite specimen is high and 

 spherical, the Great Oolite specimen low and flattened. May not these two forms 

 eventually prove to be C. sinuatiis, Leske, C, Plotii, Klein, respectively ?" 



Affinities and differences. — This large discoidal urchin is so distinct from all other 

 congeneric forms that it can scarcely be mistaken for any of them. Its nearest affinities 

 are with Clypeus Michelini, Wr., and Clypeus MiUleri, Wr. 



It is distinguished from Clypeus Michelini by its greater convexity, the wideness of the 

 poriferous zones, the size of the tubercles, and the width of the anal valley. From Clypeus 

 Miilleri, Wr., by its orbicular outline, and the absence of the produced, deflected, and 

 truncated posterior border, so characteristic of that species. From Clypeus Hugii, Ag., it 

 is distinguished by the extension of the anal valley from the disc to the border ; whereas in 

 that species a portion of undepressed test always separates the disc from the valley. 



