4 



BRITISH FOSSILS. 



of Brongniart is intermediate between it and the normal type. The 

 A, pustulosa of Lamarck, founded on a figure by Leske, should 

 surelv rather be referred to a specimen intermediate between « 

 and e, and nearest a. 



The fourth species, Ananchytes Gravesii of Desor, has never been 

 figured or described at length, but from the brief note concerning it 

 I think it may prove to be a form intermediate between /3 and d. 



The fifth, A. tuberculata, Defrance, is probably truly distinct. 



The sixth, A. semiglohus (Lamarck), seems to be an intermediate 

 form between a and s. The figure in Leske is that of a young a. 



The seventh, A. conica, Agassiz, has its extreme form in 

 pyramidata of Portlock, and its variation passing into the normal 

 type through the Ananchytes ovata figured in Part I. plate 4, 

 figs. 4-6, of the Echinodermes Suisses. 



The abundance of this Ananchytes in our Upper Chalk, and the 

 fine state of preservation in which they are usually found, has 

 enabled me to examine minutely and compare a great number of 

 specimens ; an inquiry the more desirable, as, according to some 

 naturalists, and those of high repute, there are many species of the 

 strict Ananchytes type, allies of A. ovata, whilst the experience 

 of British collectors would rather inchne in favour of the view that 

 we have but a single species, exhibiting numerous variations. To 

 this last view I feel compelled to assent, for however distinct the 

 forms termed ovata, gibha, striata, semi-glohus, conica, &c., may 

 seem when selected and contrasted in the cabinet, every one of 

 them is linked with the others by the most delicate shades of 

 gradation. Neither in degree of elevation, rotundity, flatness 

 of base, or curve of back, smoothness or roughness, can I find any 

 constant character. Scarcely two individuals out of more than a 

 hundred examples now before me exhibit the same proportions. 



The specimens vary in outline of base from ovate or oblong to nearly 

 round, and in outline of back from that of a depressed oblong 

 hemisphere to an abruptly-sided pyramid. They vary also in degree 

 of granulation of surface, in the number of primary tubercles, and 

 in the height of the ambulacral and interambulacral plates. The 

 number and proximity of pores in the avenues increase with age. 



In a well-grown adult example, above 2^ inches in height, the 

 dorso-lateral plates of the ambulacra are between 40 and 50 in 

 number, and of the inter-ambulacra about half that amount. In a 

 specimen 1 inch ^ in height the number of the ambulacral plates 

 is under 30, and of the inter-ambulacrals half that amount. The 



