366 Mr. Toulmin Smith on the Classification 



into long and wide regular cylinders either grouped near the 

 base or dividing off one from the other. 



This species differs most essentially from the last. Both pri- 

 mary and brachial fold altogether differ. The former exactly re- 

 sembles the fold of Ventriculites tessellatus. The latter is very 

 peculiar. It often displays a group of cylinders radiating out 

 from near the base just like the outstretched fingers of the hand ; 

 and the length and thickness of the cylinders increase the resem- 

 blance. Hence the name. It is often found however under a 

 modification of this form, rising to a considerable height, and one 

 branch rising out of the other, at considerable distances, as it 

 increases. In each case alike the cylindrical cavities of all the 

 branches open into each other, and there is no true separate 

 central cavity into which they open. The separate branches form 

 exceedingly regular cylinders, and the primary fold is marked 

 ' on each with perfect regularity. The margin goes off to a round 

 edge, as will be seen in the figure. 



The species cannot be at all understood, or even detected, 

 without careful clearing out with the knife and needle ; since 

 its very nature, like that of every other species in this section, 

 prevents it ever coming out of the matrix entire by any acci- 

 dental fracture : beautiful fragments of it are, however, sometimes 

 found. 



The species is found in the Upper and Middle Chalk. A form 

 essentially the same and of the same habit is sometimes found in 

 the lower chalk and in the chalk marl and greensand*. In all the 

 specimens from these latter beds which I have seen the thickness 

 of the wall is however much greater, and the diameter of the 

 branches also rather greater than in specimens from the Up- 

 per and Middle Chalk, a fact which it is interesting and import- 

 ant to notice as connected with the stratigraphical distribution 

 of these fossils, though I do not conceive these minor characters 

 sufficient to justify, at present at any rate, and until their con- 

 stancy is fully established, a distinct species or even variety for 

 those lower forms. 



Among the Mount Rhanden specimens in the British Museum 

 are individuals identical in general character and habit with the 

 last-named modification of B. digitatus. 



4. Brachiolites tubulatus. PI. XV. fig. 7. 



Membrane having a more or less slight, but close, primary fold, 

 without any regular figure : brachial fold in narrow tubes in- 

 creasing in size and length from the base upwards, and closely 

 ranged round a central cavity; each tube narrowing at the 

 mouth. 



* See ante, p. 354. 



