88 



ing directly into each other, or by opening into a central cavity, 

 which they surround. 



This genus therefore presents the Ventriculidse in a new light. 

 With far less of the intricate complexity of fold of membrane 

 which is found in the other genera, it exhibits what may, in con- 

 tradistinction, be called a convolution of membrane varying greatly 

 in different species. And that this " convolution " is a distinct 

 thing from the " fold " already noticed will be evident, should its 

 essential distinctness be not otherwise recognized, from the fact 

 that several species have the fold as well as the convolution. In 

 the descriptions which follow, the fold and the convolution will 

 be distinguished as the primary and the brachial fold. 



This genus, like each of the others, will be found to have all 

 its modifications adapted for the purpose of maintaining strength 

 and stability of form and the free access of sea-water. We shall 

 find some contrivances for these purposes of singular novelty and 

 beauty ; and it is upon the marked distinction of two groups in 

 one general arrangement for ensuring to the whole polypiferous 

 surface a full and constant supply of the grand element of the 

 existence of the creatures that the sectional division of the genus 

 is founded; the one section having the separate lobes of such 

 size, or so arranged with reference to a central cavity, that one 

 main* entrance afforded sufficient access to the sea- water; those 

 lobes in the other being so extended, or so arranged with reference 

 to each other, that additional means were needful for that end. 



The roots do not, in general, differ in this genus from the same 

 part in the others ; but I shall have occasion to call attention to 

 some special and very remarkable contrivances in the arrange- 

 ment of this part. 



The forms are all well-defined ; and though, like Ventriculites, 

 specimens of such species as are not of great rarity are found of 

 various sizes, there is little danger of confounding any two of the 

 species unless in a very fragmentary state. As to the question 

 of growth, the present genus may be considered, from the fact 

 just mentioned, to stand on the same ground as the genus Ven- 

 triculites. 



Different forms of this genus are found in the Upper, the 

 Middle, and the Lower Chalk, and even in still lower beds of the 

 cretaceous series. I believe only one, B. digit atus~\ , is strictly 



* The very beautiful arrangement of J5. angularis will be found to be one 

 of those interesting apparent (at first sight) exceptions which prove a rule, 

 the main access being through the central cavity, and the lateral perforations 

 ensuring only the full and free circulation of the water entering, by that main 

 access, the cavities of the arms. The contrivance in all the group Aperti 

 is of a very different character. See post, p. 96. 



t See Ann. and Mag. Nat. Hist. vol. xx. 1st Ser. p. 337, and ante p. 43. 



