of the Vcntriculida? of the Chalk. 365 



last being distinct*, and regular in form and arrangement. The 

 term racemosus seems peculiarly expressive of the character of 

 this species. The perfect animal rose on a rather high stem. 

 I am fortunate in possessing a specimen in flint, from which the 

 fig. 6 on PI. XV. is drawn, with the stem and roots entire, a 

 condition in which the members of the present section are very 

 rarely found. I have another specimen in which the processes 

 are very conspicuously seen, also an exceedingly rare circumstance 

 in specimens of this genus. 



The wall of the brachial fold is full a line in thickness, often 

 more, owing to the depth of the primary fold, which much re- 

 sembles that of Ventriculites quincuncialis. The central cavity 

 is usually very small, the access of sea-water being abundant, and 

 its circulation free, by means of the open short cylinders which 

 the brachial fold assumes. Occasionally the central cavity is wide 

 however ; though the opening into it is not, even then, propor- 

 tionably wide, a fact which might have been anticipated. 



It would be difficult to confound this species with B. tuberosus, 

 though the general habit is the same. The primary fold, and 

 the nature of the lobes, open at the extremity, and much larger 

 than those of B. tuberosus, at once distinguish the twof. 



All the specimens which I have seen of this species are from 

 the Upper Chalk. 



3. Brachiolites digitatus. PI. XVI. fig. 2. 



Membrane having a deep primary fold of regular quadrilateral 

 and rectangular form > usually more or less oblong, and arranged 

 in tessellated figure : brachial fold branching out irregularly 



* I have observed, in one specimen in flint, an indistinct appearance as 

 if two of the cylinders adjoined at one point. If it be a true anastomosis 

 (which I doubt), it is a very rare exception. No symptom of it has been 

 seen in any other instance. 



f Through the kindness of Mr. Wetherell I am in possession of one very 

 interesting specimen of this species, strikingly illustrative of the truth of the 

 views expressed in the early pages of these sheets as to the peculiar state in 

 which silicified specimens are found. Each of the cylinders is separately 

 encased in a thin coat of flint, so that the whole was actually taken, by a 

 distinguished palaeontologist, for the silicified fruit of a conifer. In touch- 

 ing on this subject I cannot forbear citing a passage from Humboldt's 

 ' Cosmos,' published long after my remarks on the formation of flint and on 

 the silicified Ventriculites were written, and which is in direct accordance 

 with, and therefore supports, the views advanced by me on both those sub- 

 jects. That writer alludes to the siliceous-shelled infusoria as being uni- 

 versally found in sea-water, " although the chemical analysis of sea-water 

 has not shown silica to be one of its essential constituents ; and it could only 

 indeed exist in water in a state of simple mixture or suspe?isio7i." (Vol. i. 

 p. 341.) This, however, is a state of things which Mr. Bowerbank has ex- 

 pressed himself unable, " by any stretch of the imagination," to " conceive." 

 • Ann. and Mag. Nat. Hist.' vol. xix. 1st Ser. p. 260. 



