80 



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. 



b. Dilatati. 



Head broad and rounding : plaits loose and irregular. 



All the species of the present section differ Very remarkably 

 from the Annulati. In the latter section the heads in all the 

 species were of nearly the same size relatively to the size of the 

 whole body ; as also was the central cavity. The various differ- 

 ences of contrivance by which extent of surface was gained at the 

 same time that the free access of sea-water was maintained, were 

 found in the different modes of folding of the membrane of the 

 wall. In the present section the character of the fold of that 

 membrane differs also in the different species ; but that difference 

 is accompanied by very remarkable differences in the form and 

 extent of the head. The latter becomes the most conspicuous 

 instead of a mere subordinate part to the observer of the whole 

 body. As, therefore, the difference in the heads is a necessary 

 accompaniment of a difference in the fold of the membrane 

 (though rather in the relation of consequence than cause), it will 

 simplify the labour of the inquirer if the character of the head is 

 adopted as one of specific difference. The names given have 

 therefore a reference to this point. 



It will be obvious that, the looser the folds, the more necessary 

 would become the greater extent of cephalic membrane in order 

 to secure the objects already suggested as those for which that 

 remarkable structure was designed. Hence the variations in this 

 conspicuous character in the forms immediately under consider- 

 ation. 



There are minor modifications in individuals of each species 

 which would probably be held by many to justify the assignment 

 of each species as a distinct genus ; an arrangement which would 



