354 Mr. Toulmin Smith on the Classification 



cretaceous series. I believe only one, B. digitatus*, is strictly 

 common to all the beds, and that undergoes a modification of 

 character in the lower ones. Some of the most marked forms 

 are, as far as present observation has extended, peculiar to the 

 lower of these beds. 



§ a. Operti. 

 Brachial folds closed at extremity. 



1. Brachiolites tuberosus. PL XV. fig. 3. 



Membrane having an irregular and generally slight primary fold : 

 brachial fold arranged subspirally around a wide central ca- 

 vity, and at rather distant intervals, in tuberous sacs, broad and 

 flattened at the head, with slight depressions in the middle of 

 the head. 



Of this remarkable species I am fortunate in possessing four 

 well-marked specimens, all of which were found by myself, though 

 in very distant parts of the country, and they are the only spe- 

 cimens I have ever seen. The form is striking. Rising from, 

 apparently, a very short root, it attains a considerable height, one 

 of my specimens being upwards of three inches high. The sacs 

 usually project about four lines from the central cylinder, and 

 are about four lines wide, though sometimes more, at their 

 broadest part. They open by a broad and trumpet-shaped mouth 

 into the central cavity, which is wide and open at the top. Thus 

 the access of sea-water is freely maintained. 



A transverse section gives the accompanying figure, which will 

 be at once distinguished from that seen in a similar section (see 

 fig. G, p. 289) of Cephalites capitatus or of any other of that 

 genus. pig, m. 



The figure (3. PL XV.), which is 

 from a specimen carefully developed 

 from the chalk by means of the needle, 

 gives a complete idea of the species. A 

 large part is broken away, and thus the 

 inside, as well as the outside, is cleared 

 out and displayed. Being, however, 

 developed by this means, and the spe- 

 cimen being one in which the oxide of 

 iron very greatly abounds, the primary fold is hardly to be seen. 

 A careful comparison of this with other specimens shows that 

 that fold was, like that of Ventriculites impressus, irregular ; 

 usually slight ; but occasionally deep, at any rate in that part of 

 the membrane which forms the central cylinder. 



The remark already made, in describing Cephalites campanu- 



* See Ann. and Mag. Nat. Hist. vol. xx. 1st Ser. p. 337, and ante p. 38. 



