2 



BRITISH FOSSILS. 



Echinus^ in which, according to the founder, the body is depressed, the 

 ambulacral areas very narrow, the ambulacra narrow, straight, and con- 

 taining pairs of pores ranged in a single row, the ovarian and inter- 

 ovarian plates middle-sized, and the anus covered by four valves. The 

 types of this genus, as cited by its founder, are the Echinus pustulosus 

 of Lamarck and the E. punctulatus of the same author. The genus is 

 a good one, though its characters were partially misunderstood by its 

 founder, for the ambulacral pores are not uniserial. It presents a very 

 peculiar and undescribed structure of the ocular plates. The characters 

 of Arbacia, as given by Agassiz, are, however, altogether different from 

 those just cited, for he confines it to " small subspherical urchins, having 

 the test covered by numerous Httle smooth-based imperforate tubercles, 

 ranged in numerous rows on the interambulacral arese, and sometimes 

 also on the ambulacral ones. Pores ranged in a simple series. Mouth 

 circular, without deep notches. Genital apparatus narrow and ring- 

 shaped." All the species enumerated in his synopsis are cretaceous or 

 tertiary fossils. The Arbacice of Gray are transferred to Echinocidaris 

 of Desmoulins, a genus synonymous with Arbacia, in the sense of its 

 founder's definition. In that sense the several fossils enumerated under 

 Arbacia, in the " Catalogue Raisonne des Echinides," cannot be so 

 called,* for neither Echinus monilis nor Echinus granulosus partake of 

 its characters. They are, in fact, true Echini, as I have elsewhere 

 shown, when describing the living E. monilis, and am prepared to 

 prove, with respect to the fossil E. granulosus, that neither the degree 

 of convexity of shell, or abundance of spiniferous tubercles, or the 

 arrangement of the pores (which are not in simple series), or the struc- 

 ture of the mouth or of the genital apparatus, can warrant the consti- 

 tution of a generic group distinct from Echinus, using the word in its 

 restricted sense, for the species in question. 



Description. — The body varies in shape from sub-globular (as in 

 fig. 2), to sub-conical (as in fig. 5), and to the unarmed eye resembles 

 a miniature melon, studded over by minute uniform tubercles, and 

 divided into five broad and five narrow segments, by narrow, depressed, 

 punctured furrows, radiating above from a circle of apical plates, sur- 

 rounding a central perforation, and converging below towards a much 

 larger central circular orifice. 



The ambulacral and interambulacral segments are composed of similar 

 polygonal plates, very numerous. Those of the former are shortly 

 oblong, those of the latter very narrow in vertical dimensions. All are 

 covered by small, imperforate, thick-set primary tubercles, separated by 

 minute secondary ones. The former alone were probably spiniferous, 

 the latter probably indicate the position of the bases of pedicellarige. 

 The tubercles of both the ambulacral and interambulacral plates are 



