77 



thing we have yet seen. Instead, like Ventriculites mammillaris, 

 of being mere rounded elevations on the plait, they stand out 

 prominently from it ; and a careful dissection shows that their 

 shape is generally that of a lozenge, with the acute angles in the 

 horizontal, and the obtuse in the perpendicular, line of the whole 

 body. Fig. 7* of PI. XIV. shows the manner of the projections 

 from the plait and the figure which the peculiar shape of their 

 tops causes to be seen on a clean section exactly through the 

 middle of any one. In all the specimens of this species which I 

 have seen, the projections run in nearly regular spiral lines round 

 the body. 



On the inner surface of this and of some other broadly de- 

 pressed species there is a very small and slight depression be- 

 tween each of the large depressions, and both on the plaits and 

 on the places of the united bulgings. It is barely traceable, and 

 may easily escape notice. It is however worthy of remark as an 

 additional contrivance for gaining extent of surface, and an ad- 

 ditional instance of the exhaustless variety of plan which nature 

 adopts in the development of life. 



6. Cephalites retrusus. PI. XIV. fig. 8. 



Plaits broad and very deep : outer plaits 



: inner plaits raised in rather small but very promi- 

 nent projections at regular and close intervals, and in such 

 manner that they range spirally round the whole body, and 

 quincuncially relatively to each other : projections cylindrical, 

 rounding off slightly at the top and with an exactly central 

 and rather deep circular depression (sometimes two) on the 

 top of each projection : wall very thick. 



This form departs from every other which has been named. 

 It is the first and only instance in which we find projections on 

 the inner plaits, which have been already more than once found, 

 and will be so again, on the outer plaits. The fold which marked 

 the outer plaits of C. bullatus is here found, with striking modi- 

 fications however, on the inside. The projections are much 

 smaller and closer than in that species, but no less prominent ; 

 while each one is again marked by a deep though small and ex- 

 actly central depression. It is altogether a very extraordinary 

 formf- In chalk specimens it would at once be distinguished 



* In this figure I have connected the inner and outer plaits by brackets, 

 the outline of each merely being given for the sake of clearness. 



f Forms like this afford very strong ground of caution against the hasty 

 adoption of any development theories. The whole of the present subject af- 

 fords, indeed, the strongest ground for such caution. We see infinite variety 

 all subservient to the ends of life ; and throughout which one Unity is 



