of the Ventriculidse of the Chalk. 287 



each place, these regular marks are left, contrasting strongly with 

 the portions of the membrane adhering to the matrix elsewhere*. 

 Fig. 16, which should be compared with fig. 14 of PI. XIII., 

 shows the peculiar elevations on the plaits : and fig. 15 is a trans- 

 verse section showing three plaits ; the uppermost being struck 

 at a point where there is not any projection, the two others just 

 at the bend of two projections. 



The specimens which I have distinguished as a variety, under 

 the name of annulatus, appear to be cases in which the horse-shoe 

 elevations have become more than usually continuous both on the 

 same plait and by anastomosis with those on adjoining plaits. 

 This character is sometimes seen on the lower part of specimens 

 the upper part of which exhibits the true normal characters of 

 C. catenifer, as in fig. 9. PI. XIV. In some cases, however, the 

 same appearance of connected rings, instead of rows of open 

 links, covers a large part, or the whole, of the surface ; and it is 

 important that the true place of such specimens should be un- 

 derstood, whence the utility of distinguishing them as a variety of 

 C. catenifer. 



It generally happens that, even in the most characteristic spe- 

 cimens of this variety, there are places in which the projection 

 on the outer plait stands, as it so often does in the normal C. ca- 

 tenifer, single and wholly unconnected with any other projection 

 on the same or on any adjoining plait. In that case, instead of 

 being horse-shoe shaped, the circle is usually complete. We thus 

 find, on an external plait, a fold very similar to that which cha- 

 racterizes the inner plaits of C. retrusus. 



8. Cephalites compressus. PI. XIV. fig. 10. 



Plaits broad and very deep : outer plaits projecting prominently 

 in very elongated loops often linked at one extremity and en- 

 larged at the other : inner plaits often inclining towards, and 

 anastomosing with, adjoining plaits : pouch very short : pro- 

 cesses very conspicuous : wall very thick. 



This appears to be quite a distinct species from the last. Its 

 fold is looser, approaching therein to the character of the group 

 Dilatati. The external modification of fold is very different 

 from that of C. catenifer; while the internal difference is even 

 more marked. Instead of depressions we have here anastomosis 

 with the adjoining plaits; and at the places of anastomosis 

 the figure becomes almost angular, instead of circular as here- 

 tofore. 



* See ante, p. 209, note f. 



