51 



plained, are a large number whose general form is that of a more 

 or less open or close sac, the wall of which rounds or thins off to 

 a marginal edge. All of this kind are single, and supported on 

 a single root, unless in those few abnormal cases before men- 

 tioned*, and which afford no exception to the principle either of 

 the structure or classification. Where, as very rarely occurs, two 

 are united, it is at the roots that they are united. They are 

 not branches of one body. 



All these forms I distinguish by the name of VENTRICULITES. 



Next to these are naturally placed another group, all the mem- 

 bers of which are much rarer than the last, most of them of great 

 rarity, but yet exhibiting a diversity of forms as great, well- 

 marked and constant as the different individuals of the genus 

 Ventriculites. All however are marked by the very striking 

 peculiarity of the wall of the pouch not thinning or rounding 

 off to a marginal edge, but being crowned by a broad and distinct 

 head, prominent and well-defined, and totally differing in aspect, 

 structure, and function from the rest of the body. This charac- 

 teristic suggests, as peculiarly appropriate, the generic appellation 

 of CEPHALITES. 



The two genera thus distinguished each exhibit, though with 

 striking modifications, more or less of the simple pouch form in 

 their internal cavity, or of obvious singleness in the general shape 

 which the fold of the wall of their cavities, or their apolypous 

 head, assumes ; but a large group remains to which neither cha- 

 racter applies, and all the members of which stand out conspi- 

 cuously as folded in many lobes and in many broadly separated 

 parts. The word brachium being often used by the best authors 

 in. the sense of projection simply, I use the diminutive of that word 

 to distinguish all of this group by the name of BRACHIOLITES. 



But, again, the individuals comprised within the description 

 of the genus Ventriculites are found to exhibit two broad modi- 

 fications in the general aspect of the membrane composing the 

 wall of the pouch. The two sides of the wall correspond in the 

 one group, both surfaces being either smooth, or, if marked with 

 folds, the depression of one side having a corresponding eleva- 

 tion on the other; in the other this correspondence is absent, 

 owing to some change in the direction of the fold before reaching 

 the opposite surface, as already alluded tof- It will materially 

 assist the memory and researches of the inquirer if we accord- 

 ingly divide the genus Ventriculites into two sections, which I 

 distinguish by the names Simplices for those species having cor- 

 responding surfaces, Complicati for those which change the direc- 

 tion of their fold between the two surfaces. 



* Ante, p. 18. f dnte, p. 16. 



D2 



