n THE CRAG POLYZOA. 



distinct wall. Each layer of cells being thus readily separable from the other, and not 

 intimately united, as in Eschara. The dorsal surface of each layer, when they are sepa- 

 rated, is smooth and longitudinally sulcate, or like a ridge and furrow roof, the ridges 

 and furrows of the opposed surfaces fitting into each other. The longitudinal lines of 

 cells are also easily separated laterally, though perhaps not quite so readily as in the other 

 direction. 



No description or figure of the recent species which was procured by Mr. 

 M'Gillivray, in the voyage of the " Rattlesnake," has as yet been published. I have, 

 therefore, given in PI. I, fig. 1, magnified figures of portions of it, for comparison 

 with similarly magnified figures of the fossil form (fig. 2). In the ' Paleontologie 

 Fran^aise,' M. D'Orbigny describes and figures sixty fossil species of Bijlustra, a number 

 which doubtless very far exceeds that of the truly distinct forms. Among these, none are 

 foliaceous or expanded, all being either subcylindrical and closely allied to Vincularia, or 

 compressed and ramose, many of which might perhaps, with equal justice, be referred to 

 Eschara. In the figures no indication is afforded that the two layers are readily separa- 

 ble ; but as M. D'Orbigny includes this peculiarity in the generic character, it is to be 

 presumed that the species depicted do possess it. 



The genus consequently may be subdivided into two groups. 



(a) Polyzoarium foliaceous, expanded, subcylindrical, compressed. 

 {b) Polyzoarium ramose. 



The genus, as above defined, would seem to have commenced in the Cretaceous 

 period, during which it attained its greatest development, the species afterwards appearing 

 to become isolated and far less numerous up to the recent epoch, when they seem to have 

 dwindled down to one or two. 



1. B. DELiCATULA (^^. sp.) PI. I, figs. 2 and 4 ; PI. II, fig. 7. 



Cellulis quadrangularibus, oblongis, supra arcuatis ; septis, laminaque granulosis ; 

 apertura ovata, suborbiculari sen elliptica; dentem serratum internum infra ostendente. 



Cc41s quadrangular, oblong, arched above ; septa and lamina granular ; aperture ovate, 

 suborbicular, or elliptical ; a serrate denticle within the lower margin. 



Habitat. — C. Crag ; occurring abundantly in large irregular fragments, S. W., J. S. B. 

 {Becent), Australia, Manilla? 



