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



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

 By J. Toulmin Smith, Esq. 



[Continued from p. -48.] 



Descriptions of sjjecies. 

 Family Ventriculid^. 



Character. Structure. Polypidom membranous : membrane com- 

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



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

 ihickness and of superficies, and connected at all their angles 

 ly other fibres having a regular octahedral arrangement : exte- 

 "or to this, both within and without, a dermis composed of a 

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



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

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



hedral fibre. 

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



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

 xtending below into radicles ; never affixed to solid bodies. 



The details which have been already given render further ob- 

 rvations on the characters of the family unnecessary. 



Genus Ventriculites. 



Character. Pouch-shaped j 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 piirpose of securing permanence of form, and thus safety and 

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

 effected 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 

 iBasily 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 tlie 

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

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



