Mr. Toulmin Smith on the Ventriculidse of the Chalk. 203 



XXIII. — On the Vcntriculidse of the Chalk; their classification. 

 By J. Toulmin Smith, Esq. 



[Continued from p. 18.] 



Descriptions of species. 

 Family VENTRicuLiDiE. 



Character. Sti'ucture. Polypidom membranous : membrane com- 

 posed internally of fibres arranged in several — usually five* — 

 layers of cubic squares, equal, for the most part, in the plane of 

 thickness and of superficies, and connected at all their angles 

 by other fibres having a regular octahedral arrangement : exte- 

 rior to this, both within and without, a dermis composed of a 

 single layer of smaller squares, and in which the polyps are 

 lodged, usually on both surfaces : exterior to this a simple epi- 

 dermis : roots distinct, less regular in structure and without oc- 

 tahedral fibre. 



Habit. One or more central cavity, the principal opening to 

 which is at the top : roots ensheathing base of polypidom and 

 extending below into radicles ; never affixed to solid bodies. 



The details which have been already given render further ob- 

 servations on the characters of the family unnecessary. 



Genus Ventriculites. 



Character. Pouch- shaped ; varying greatly in size and dilata- 

 tion : cavity single and regular : membrane forming the wall 

 of the cavity either simple and smooth on both surfaces or 

 more or less closely and regularly folded (thus giving it a ru- 

 gose character) : margin of wall thinned or rounded off* to an 

 edge : polypiferous on both external and internal surface. 



The structure of all the Ventriculidse is obviously designed for 

 the purpose of securing permanence of form, and thus safety and 

 free access of water to all the individual polyps. This object is 

 eff'ected by two means : first by the very remarkable structure of 

 the membrane already described ; secondly, by the regular, often 

 nearly hemispherical figure which the whole body assumes. 



Specimens of Ventriculites are found of all sizes, and it can 

 easily be understood that, from the earliest period of their deve- 

 lopment, the same, general form is assumed, which, as they increase 

 and spread, is still retained. Thus the question of their growth 



* I believe it to be always five ; but the difficulty which exists, from the 

 causes already named, in ascertaining these minute points in all individual 

 instances induces me thus to qualify the generality of this character. 



