99 



b. Aperti. 

 Brachial folds open at extremity. 



1. Brachiolites foliaceus. PL XVI. fig. 1. 



Membrane simple and without any primary fold : brachial fold 

 variously winding and irregularly anastomosing, and thus 

 forming irregular but close and connected sinuous cavities, with 

 rounded, but irregularly arranged, external openings : mass 

 rising to a considerable height ; expanding slowly from root, 

 and the whole maintaining, throughout, a narrow diameter. 



The style of fold of this delicate species is very similar to that 

 of the recent Eschara foliacea. The form assumed by the whole 

 mass differs however materially. Instead of spreading horizon- 

 tally, the habit of B. foliaceus was to rise perpendicularly. It 

 sometimes attained six inches in height, but seldom more than 

 an inch in width at its broadest part. The external openings in 

 this species also differed materially ; those openings being usually 

 separate, and circular or oval in form, not irregularly running 

 into each other as in E. foliacea. 



Fragmentary portions in flint or chalk may readily be distin- 

 guished from those of any preceding species by the greater deli- 

 cacy of the membrane and closeness of the brachial fold. The 

 upper part of fig. 1. PL XVI. shows the appearance displayed on 

 a vertical section ; while the lower part of that figure shows the 

 external appearance of the fossil when entire. 



The close anastomosing of the brachial folds of this species must 

 have given great strength to the whole body ; while the freely 

 communicating cavities would allow constant access and circula- 

 tion of the sea-water. 



I have a specimen in which the whole animal part is converted 

 into iron pyrites and the cavities are perfectly clear of all matrix. 



The species is found in Upper and Middle Chalk. 



2. Brachiolites racemosus. PL XV. fig. 6. 



Membrane having a rather deep primary fold, round, and of 

 nearly equal width the whole depth of the fold, and arranged 

 in quincuncial figure : brachial fold beginning at some distance 

 from the base, and running in narrow and short but regular 

 cylinders ranged subspirally round a small central cavity at 

 rather distant intervals. 



This species differs essentially, from the last in having a deep 

 primary fold ; in having a distinct central cavity, though varying 

 in size, into which each of the brachial folds opens ; and in these 



G 2 



