133 



Lamarck, who considers that the characters of the am- 

 bulacra should be reckoned among the distinguishing generic 

 characters, has instituted the following genus for the re- 

 ception of such of the echinidae as accord with the preceding 

 genus, cassidulus, in every respect, except in the state of 

 the ambulacra. These, in this genus, reach only to the 

 margin ; but those which belong to the following genus are 

 extended to the centre of the base. 



GENUS V. Nucleolites. Ovate or subcordiform, rather 

 raised ; the mouth beneath and subcentral ; the vent at the 

 end, much above the margin ; the ambulacra radiating to 

 the centre of the base. 



Synon. Echinobrissus, Breyn. 



Sp. 1. Nucl. scutata. Elliptical and rather quadrilateral ; 

 slightly convex and widest behind, where it is sulcated; 

 vent dorsal. Fossil. 



Lamarck places under this species, spatagus depressus 

 of Klein. This is an interesting fossil. From its form and 

 sulcus, and from the situation of the mouth and vent, it ap- 

 proximates to clypeus ; and, from its four radiating ambu- 

 lacra, with the fifth passing along the sulcus, it resembles 

 spatagus. 



2. N. colombaria. Obovate, rather elevated, and widest 

 behind ; ambulacra with biporous lines, with faint striae . 

 the mouth pentagonal. Fossil. 



3. N. ovulum. Ovate and pulvinated ; with thinly scat- 

 tered tubercles with surrounding rings ; the ambulacral lines 

 subbiporous. Fossil. 



4. N. amygdala. Ovate and rather gibbous, the vertex 

 rather prominent ; the ambulacra very narrow ; the vent 

 beneath a small projection. Fossil. 



A figure illustrative of this genus appears to be unneces- 

 sary, as it would differ from that of the preceding genus 

 only in the form of the ambulacra. 



