81 



indeed be far better warranted than many such divisions both in 

 recent and fossil classifications. It does not seeni to me how- 

 ever that the principles of a sound classification will, in the pre- 

 sent state of our knowledge, justify such an arrangement*. 

 Moreover, all the species of the present section are of extreme 

 rarity ; so rare, that it is very probable that few even diligent col- 

 lectors will succeed in obtaining specimens of each, unless some 

 bed abounding in them, and at present unknown, should be dis- 

 covered. 



1. Cephalites capitatus. PL XIV. fig. 11. 



Plaits very deep ; dividing longitudinally, and so reduplicating, 

 very constantly, as they pass from the inner to the outer sur- 

 face; points of anastomosis at irregular distances on both 

 inner and outer surfaces : central cavity small : head rounding 

 and very wide : wall falling in very rapidly but in a regular 

 slope from outer margin of head to root : diameter of- whole 

 body greater than its height. 



In some specimens of the present species the plaits are very 

 traceable on the outside ; in others much less so, on account of 

 the almost total absence of oxide of iron. In each case, however, 

 it is equally obvious that the number of plaits seen on the outer 

 surface is given by the longitudinal division and reduplication of 

 the plaits towards that surface, in the same 

 way as the increase of plaits from base to margin 

 has already been described as being effected 

 by a transverse division and reduplication f. 

 The accompanying figure will explain the pre- 

 sent mode of this reduplication. This arrange- 

 ment takes place to some extent in most of 

 the Annulati, but the very small size of the 

 central cavity in C. capitatus renders this peculiarity constant in 

 this species, and one of its most marked characteristics. 



The general form of this species is so peculiar that a vertical 

 section through the fossil displays a triangular figure, of which 

 the base of the fossil forms an obtuse angle, while the external 

 margins of the head form acute angles with the wall. It is thus 

 impossible to confound this species with C. compressus, as the 

 wall of that species, like that of every other species of the sec- 

 tion Annulati, usually forms, inside and outside, nearly a right 

 angle with the headj. 



* See ante, p. 46 note, and pp. 47, &c. f See ante, p. 64. 



J I have an interesting specimen of this species in which two individuals 

 are close together ; actually touching. But they cannot be mistaken for an 

 example of C. constrictus, each individual having separate roots or places of 

 roots, (see before, p. 51,) and not being parts of one single body. 



F 



