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LETTER IY. 
PLEUROCYSTI, ECHINARACHNIUS COR MARINUM, SPATANGUS, AND 
ITS SPECIES. 
T he third grand class of Echini is the Pleurocysti, 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, Leslc. 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 spatangus. 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 con- 
struction, as well as in their having a bilabiated mouth, and being 
without teeth. The first family is formed of cordated spatangi, with 
a sulcated vertex. The first species is, Spatangus, cor anguinum, 
Tab. xxiii. c. d. et Tab. xxiii. 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 arese, by as many grooved am- 
bulacra, formed by four rows of pores, connected by transverse lines. 
