On Cyrtotna, a new genus of Fossil Echinida. 175 



when it is oval, &c. From these irregular forms we again 

 return by means of the Ananchites and Spatangi, which 

 differ from the Scutellae, and other genera connected by 

 direct affinities, to the star-fish, in presenting no dis- 

 tinct rosette on the lower surface of the body ; nothing 

 in fact equivalent to the fissures of the Asterias is to be 

 observed in these genera. In Spatangus pilosus, and those 

 lengthened oval species, particularly Echinonaus lampus,* 

 we think a change of character may be observed to the 

 Holothuriae, while the flattened form of Spatangus bufo 

 evinces the return of the affinities again to the star-fish ; but 

 this last remark I merely suggest as probably correct. The 

 very singular form Echinonaus lampus of De la Beche, 

 found by that geologist in the green sand near Lyme, 

 however unquestionably evinces a tendency of nature to 

 change her form from the spheroidal incrusted Radiata, to the 

 soft and elongated Polypes. From the genus Echinus, which 

 may be said to be the culminating point of the former, 

 we trace the change from the strong, unyielding bony 

 spheroid, constructed with such wonderful precision of hun- 

 dreds of thick pieces symmetrically arranged, to the vari- 

 ous forms of Spatangi, in which the testa becomes thin- 

 ner, and more various in shape, until its strength and 

 spheroidal form is gradually lost. In Echinolampus an- 

 gulosus, the recent species that approaches nearest to M. 

 De la Beche's fossil already alluded to, we find the disc 

 slightly elongated between the two posterior ambulacra, a 

 change which is first perceptible in the Spatangi as well as 

 most of the other groups which have lost the rosette on 

 the lower surface, and have here been placed after the 

 Echini, until at length we find the fossil Echinolampus 

 present an almost complete transition to the Holothuria, 

 which are elongated, soft radiated animals, in which the 

 place of an external skeleton is supplied by a highly 

 organised coriaceous skin. 



* Transac. Geol. Soc. New Ser. 1, plate in. figs. 3, 4, 5, 



