LETTER IV. 



PLEUROCYSTI, ECHINAR ACHNIUS COR MARINUM, SPATANGUS, 



AND ITS SPECIES. 



1 HE third grand class of Echini is the Pleitrocysti, in which the vent of 

 the animal is on some part of the side, or on the upper surface. The 

 only genus of the first section, and which also contains but one species, 

 is Arachnoides, Klein. Echinarachnius, Lesk. so named from its yielding, 

 by its markings, the appearance of a spider's web. It is not known 

 fossil. Of the second section, Cor marinum, or sea-heart, characterized 

 by the bilabiated mouth being in the third region of the axis of the base, , 

 and the anus in the side of the truncated extremity, the first genus is 

 spatangm. In this genus, or, as he terms it, family, Leske, with Muller, 

 includes spatangus, spatagoides, brissus, and brissoides, not considering the 

 absence of the groove to be a generic distinction, and finding that the 

 animals agree in their general construction, as well as in their having 

 a bilabiated mouth, and being without teeth. The first family is 

 formed of cor dated spatangi, with a sulcated vertex. The first species is, 

 Spatangus, cor anguinum, Tab. xxiu. c. D. et Tab. xxm. C. Klein, cor 

 anguinum Anglicum. Plate III. Fig. 11, is a variety of this species. The 

 characters of this shell are, its being cordated, and more or less oblong ; 

 the base, in some, flat, and in others rather convex ; being beset, in this 

 part, with miliary, and, in the superior, with fewer and smaller granulae. 

 The back is convex, and divided into five areae, by as many grooved 



