FROM THE UPPER GREENSAND. 259 



in the British Museum. Sir Heury called it Lampas, from its i-esemblance to an ancient 

 lamp when held with the base uppermost. Lamarck, 1816, described a Clypeaster as C. 

 ovi/ormis, from the South Sea, collected by Peron and le Sueur, and referred " la variete 

 que se trouve fossile dans les vignes aux environs du Mans" to the same species. Erom the 

 angular character which the posterior half of the test exhibits Agassiz called it trilobus. 

 D'Orbigny, finding that Lamarck had noticed the fossil species from Mans as a variety of 

 C. oviformis, has given this name to the fossil, which is quite distinct from the living 

 form. I have, therefore, followed my friend Professor Desor, and retained Sir Henry 

 De la Beche's most appropriate name. 



The test is oval or oblong, obtusely rounded before, hollowed out on the sides, and 

 prolonged into an abruptly truncated rostrum behind ; it is very convex, and inflated on 

 the upper surface, its profile forming a regular curve, which is a little more depressed 

 behind the vertex than before (fig. 1 a). The ambitus is very angular (fig. 1 a, b) in its 

 posterior half, and the two lateral and one posterior lobe gives value to the name trilobus 

 which was proposed for it. The single inter-ambulacrum is much prolonged, and on it 

 two carinse are developed, which proceed from the apical disc to the sides of the 

 truncated border (fig. 1 b, c), and impart a still more angular appearance to the test. 



The ambulacra are largely petaloid on the upper surface (fig. 1 b). They are 

 contracted at the ambitus (fig. 1 d, e), and are again largely developed and petaloidal at 

 the base (fig. 1 c). 



The poriferous zones are well developed and visible throughout in the petaloidal 

 portion on the dorsum ; the pores in the external row are elongated, and in the internal 

 row round ; at the ambitus they are remote and microscopic, and in the base they again 

 become largely petaloidal, where they surround the mouth ; the petals here are distin- 

 guished hy their elegant forms and complicated structure ; the pores are increased in 

 number, and set in oblique pairs on the sides of the petals, and in the centre of each is a 

 longitudinal enlargement like the stem of a leaf (fig. 1 c). This remarkable structure is 

 shown magnified two diameters. 



The large plates on the upper surface have several rows of small tubercles, which 

 become larger and less numerous at the base (fig. 1 (/) ; besides these a fine close-set 

 granulation covers the surface of all the plates. 



The apical disc is very small, so that the lanceolate ambulacra meet close together at 

 the vertex, which is slightly excentral ; there are four perforated genital plates, with a 

 small spongy body in the centre (fig. 1 //). 



The vent is large, transversely oval, and opens near the border of the infra-marginal 

 portion of the rostrum (fig. 1 c). 



Affinities and Differences. — This fine Urchin is distinguished from its congeners by its 

 elevated upper surface, angular ambitus, prolonged rostrum and hollowed-out sides, by its 

 rostral carinse, and the remarkable pentapetaloid arrangement of the pores around the 

 peristome. 



