288 Mr. Toulmin Smith on the Classification 



^ 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 

 indeed be far better warranted than many such divisions both in 

 recent and fossil classifications. It does not seem 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. PI. 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 

 * See ante, p. 41 jiote, and pp. 42, &c. 



