FROM THE LOWER CHALK. 321 



■deeply impressed into an almost acute fiuTrow. Professor E'orbes found this abnormal 

 form at the junction of the Chalk-Marl and Upper Greensand, near Abinger, in Surrey; 

 and there is a very fine and large specimen of it in the British Museum Collection. 



I have a large specimen of this Urchin in my cabinet, collected from the Planer at 

 Rheten, which measures in length 2jj%th inches, in breadth 2^oth inches, and in 

 height 2 inches. 



Bescnption. — This vi^ell-knovifn Urchin of the Lower Chalk has long been familiar to 

 collectors of Cretaceous fossils. It was very fairly figured by Leske in his 'Addita- 

 menta ad Klein.,' and has kept its place well in the list of synonyms ever since. 

 Lamarck, however, appears to have mistaken this fossil, as he refers to Leske's figure a 

 specimen from Grignon pros Versailles, where the formation is Tertiary ; so either an error 

 in the species, or a mistake in naming the locality, has been committed. 



The test is large, subcircular, or cordiform, almost as wide as it is long (fig. 1 a) ; the 

 body is dilated and slightly grooved anteriorly, subacuminated and obliquely truncated 

 posteriorly (fig. 1 b, c). The upper surface is convex and the ambitus uniformly inflated 

 (fig. e, d) ; the base is likewise convex, except near the anteal depression for the mouth 

 (fig. 1 b). The ambulacral summit is nearly central (fig. 1 a). 



The antero-lateral ambulacra are lanceolate, apetaloid, and curve gently backwards 

 and inwards, forming a flat Gothic arch over the anterior surface of the test (fig. 1, a). 



The postero-lateral pair are lanceolate, apetaloid, and straight (fig. 1, d). The 

 poriferous zones in the two pairs of areas are very much alike. The pores of each pair 

 are set well apart, and connected by a furrow, but below the middle and in the wider 

 plates they become smaller and closer. There are about forty pairs of holes in each 

 avenue of the anterior pair, and a lesser number in the avenues of the posterior pair 

 (fig. l,y, h). 



The single ambulacrum has a special structure. It is lodged in the anteal sulcus, 

 which is wide and shallow. Its surface is concave and smooth, and it is linear-lanceolate. 

 The apices of the three anterior ambulacra converge at some distance from those of the 

 posterior pair (fig. 1, a), which meet near each other at the posterior portion of the apical 

 disc. The poriferous avenues of the single ambulacrum are very narrow, the pores quite 

 microscopic, and fewer in number than in the pairs (fig. 1, //). 



The inter-ambulacral areas (fig. 1 a, c) are formed of large wide plates, which have 

 their surface covered with a close-set microscopic granulation ; and arising in their midst 

 are a series of three or four irregular horizontal rows of primary tubercles. I have given 

 a most accurate drawing of this structure in fig. 1 (/, which shows two inter-ambulacral 

 plates and a portion of an ambulacral area magnified three times ; in fig. \ h a portion 

 of the single ambulacrum magnified three times is shown ; and in fig. 1 i is given a view 

 of three primary crenulated and perforated tubercles magnified, with the circles of 

 granules surrounding the areolae and filling in the intervening space with moderately 

 sized granules. 



