BRITISH FOSSILS. 



3 



of the back varies greatly in declivity in the several varieties ; the posterior 

 interambulacral space declining in one set of forms, becoming elevated 

 in another ; the former being the gihhus of Lamarck and the variety 

 figured under the same name by Goldfuss, the latter being the rostratus 

 of Mantell, cordatus of Lamarck. But whether declining or elevated, 

 the outline of the posterior interambulacral space is always an arcuated 

 curve. According to its elevation, so is the altitude of the anus on the 

 truncated posterior extremity. This aperture is circular, occupying a 

 depression bounded by a narrow and defined circle of smooth- edged 

 plates. The surface below it is flattened or slightly hollowed out, the 

 bounds of the depression corresponding to two terminal bosses or caudal 

 projections, which in the living animal doubtless bore two projecting 

 tufts of long spines. These caudal projections are included within the 

 sub-anal circumscribed space ; the circumscription (which in specimens 

 of this species is not always conspicuously manifest) is transversely ob- 

 long and inclined to quadrate. The ventral surface is slightly tumid, 

 but sometimes nearly flat centrally. The mouth is placed very near the 

 anterior extremity, at the inferior termination of the deeply impressed 

 anterior ambulacrum. It varies slightly in position in different speci- 

 mens. Its projecting lip is strong and thickened, and varies in degree 

 of projection. 



The dorsal ambulacra, or rather stellate portions of them, are through 

 a great part of the length of them parallel-sided, not contracting much 

 towards their lower extremities, so that their form is oblong-lanceolate, 

 and but slightly petaloid. They are all, throughout the greater part of 

 their length, placed in depressions of different degrees of depth in dif- 

 ferent examples ; sometimes very shallow, and the ambulacra almost 

 on a level with the surface, especially in young specimens. The an- 

 terior or odd one is very variable in its degree of development, and 

 differs from the others — its ambulacral area simply granulated, 

 without tumidity of the separate plates : in many examples the grooves 

 connecting its pores are obsolete. The lateral ambulacra vary greatly 

 in length in different specimens and varieties : the antero-laterals are 

 always longer than the postero-laterals, sometimes considerably so. 

 The latter are longest in the conical forms of the species, but the 

 relative length of these ambulacra cannot be taken as a specific 

 character, nor can the number of the pairs of pores in them, seeing that 

 almost every specimen varies in this respect, as may be seen by com- 

 paring the enumeration in the table given hereafter. The lateral am- 

 bulacra, however, in other points exhibit excellent characters, constant 

 throughout all the individuals of every variety of the species. Their am- 

 bulacral plates are invariably tumid, rugose, and separated by a strongly 

 marked suture, whilst the general suture running down the centre of 

 the ambulacral area is deeply depressed and groove-like. The ridge 



