of the Ventriculidse of the Chalk. 357 



brachial fold which attracted and retained the siliceous fluid in 

 its descent towards the base. Fig. N is 

 a small specimen of this kind, instances 

 which are not uncommon in some places. 



Fig. 5. PI. XV. represents a specimen 

 which has been longitudinally divided, 

 and has afterwards had the convolutions 

 cleared out by the needle. The white 

 central parts, round the entire edges of 

 which the structure is seen, are the only 

 parts where the true central cavity is cut, 

 notwithstanding the section. The depth 

 of the convolutions will be clearly seen. 

 It will also be observed that there are several places where the 

 flattened convolutions do not unite, thus leaving ample means 

 for the free admission of sea-water to bathe the whole inner 

 portion of what is really the external surface of the membrane, 

 though so much surrounded by the overspreading flattened, and 

 thus actually outer, surface. The true inner surface is bathed 

 by means of the access of water through the upper part in the 

 usual way. 



Specimens vary greatly in size. I have them from an inch to 

 at least eight inches in height. The height to which the flat- 

 tened outer surface is continuous varies in different specimens, 

 being, as might be anticipated, greater in large ones. It is some- 

 times much greater on one side than on the other, a circumstance, 

 however, which, in the living creature, would not at all interfere 

 with the free access of the sea-water, inasmuch as the communi- 

 cation was free all around within and under this expanded con- 

 tinuous surface. 



The species is found both in Upper and Middle Chalk. 



4. Brachiolites annularis. 



Membrane exceedingly fine in texture, having a primary fold of 

 minute and corresponding depressions arranged in more or 

 less exact quincuncial figure : brachial fold expanding very ra- 

 pidly into a varying number of arms opening into a central 

 open cavity ; each arm having the two walls parallel and flat ; 

 closed at upper and lateral edges ; terminating at the external 

 angle in a broad triangular lip depressed in the middle ; and 

 having at regular intervals, along its lateral edge, complete 

 perforations, between which distinct and single root-fibres are 

 attached to the membrane at intervals from the base upwards. 



This is certainly the most extraordinary and interesting of the 

 whole family of Ventriculidac. It presents some points of such 



