121 



Cassidulus is the name of the genus proposed by La- 

 marck for the reception of those echinites which Leske 

 described as having the mouth in the centre of the base* 

 and the vent above the margin, possessing, as it were, those 

 generic characters which would form a genus between 

 echinanthus and spatangus. 



Nucleolites. The echinites of this genus are admitted, 

 by Lamarck, to agree so closely with those of the preceding 

 genus, that they might be united; the only material 

 difference being, that the ambulacra in the cassiduli are 

 contracted, but, in the nucleolites^ are radiated to the 

 mouth. 



The genus cidaris is divided, by this naturalist, into two 

 genera. The one, named by him echinus, is intended to 

 comprise all those species which have the tubercles, on 

 which the spines are placed, entire ; the other, distinguished 

 as cidarites, contains all those whose tubercles are perforated 

 for the transmission of what is considered to be a muscular 

 cord. The species of this latter genus he divides into two 

 families, the turbans and the diadems. 



In forming the following arrangement, the classification 

 of Lamarck being the latest, and having the sanction 

 of a name so justly celebrated, has been almost exactly 

 followed. 



From the situations of the mouth and of the vent are 

 formed the chief generic distinctions. When the mouth 

 is central, the echini are said to be emmesostomous ; and 

 when towards the margin, apomesostomous : when the vent 

 is in the vertex, they are distinguished as anocystous, 

 when in the base as catocystous, and, when in the side, as 

 pleurocystous. The more obvious circumstances dependent 

 on the ambulacra, and on the general form and surface, 

 complete the characters of each genus. From the more 

 subordinate distinctions of form, &c. result the several spe- 

 cific characters. 



