1-23 



Genus 1. Echinus — With a roundish, testaceous, or 

 crustaceous internal skin, indining to oval ; the mouth be- 

 neath, and central, armed with five ossiculae ; the vent ver- 

 tical. Ten areas, five large and five smaller, set with im- 

 perforate tubercles for the articulation of spines ; and ten 

 porous ambulacra, or bands, interposed between the areas, are 

 disposed vertically from the mouth to the vent. Pl.ii. fig. 1. 



It has been thought advisable to follow Lamarck, in di- 

 viding the cidaris of Klein, Leske, &c. into two genera, 

 echinus and cidaris : the former having the tubercles entire, 

 and the spines which are set on them moved only by the 

 muscular fibres in the investing cuticle ; the latter having 

 them perforated through their centre, for the passage of a 

 muscular cord, which, being attached to the base of the 

 spines, serves to augment and direct their motions.* 



The echini are distinguishable, not only by their im- 

 perforated tubercles, but by the spines which are articulated 

 on these tubercles, which are simple in their forms, and 

 either smooth or very finely granulated. In some, which 

 have somewhat of an oval form, the spines are of different 

 sizes and shapes on the same shell ; these are considered 

 as approximating to cidaris. The ambulacra in the echini are 

 not so regularly formed, nor so distinctly separated from the 

 areas, as in those species which belong to the other genus. 



Sp. 1. E. esculentus. — Subglobose, rather hemispherical ; 

 areas, with tubercles not large. The ambulacra with six 

 rows of pores, disposed obliquely ; the spines acicular. 

 Recent and fossil. 



2. E. saxatilis. — Suborbicular and rather depressed ; eight 

 rows of tubercles in the larger areas, the third from the 



* It must be observed, that the difference which has been as- 

 sumed here as a generic characteristic by Lamarck, may be found 

 in the individuals of another genus. Spatangus purpureiis has its 

 tubercles perforated. 



