74 



the wall thicker. The depressions on the inside also are generally 

 smaller, closer, and more regular than in that species. The head 

 alone is sufficient to distinguish the two at a glance. 



This is the only species of Cephalites in which the longitudinal 

 fold remains unmodified on the outer face. Hence its specific 

 name. A transverse section of it is seen on fig. 1 of PL VII. 



It is a rare and delicate species : indeed all the species of the 

 present genus are rare. They do not seem to have abounded in 

 the older seas of the Middle Chalk as the Ventriculites did in the 

 Upper Chalk. Though thus rare, however, their modifications are 

 not the less clearly marked. 



In regard to the head it is proper to remark, that while, 

 throughout the present division of this genus, its breadth will 

 always be found a very near approximation to that of a transverse 

 section of the plaits, there is a slight variation in this respect in 

 individual specimens. The head often slopes a little outwards, so 



that a section of the entire body presents an 

 outline as in fig. F, in which a b is the sec- 

 tion of the head. The outline of the head 

 is always quite as sharp and well-defined as 

 in this figure. The relative arrangement 

 and proportions of the head and the plaits 

 are such that specimens of this division can 

 never be confounded with any belonging to 

 the section Dilatati. It is very rarely in the 

 present division that there is any rounding, 

 or departure from the nearly flat character of 

 the head; a character, on the other hand, 

 never present in the Dilatati. 



It is proper to notice that, in every species of this genus, in 

 order to give full strength to the head, the depressions, bulgings, 

 and other modifications of the fold, where it does not rise, as 

 in C. campanulatus, in a simple form, are so arranged that the 

 membrane of the inner wall, where it adjoins the head, is always, 

 and that of the outer wall most frequently, expanded by a lateral 

 bulging of the plait, so as for the adjoining plaits to meet just 

 at the point of union of the wall with the head. Thus the 

 whole of the inner, and often of the outer, edge of the head is 

 continuously attached to the wall, an arrangement of much im- 

 portance. On this inner edge the membrane often rises up in a 

 narrow and slightly prominent ridge above the otherwise smooth 

 surface of the head. 



2. Cephalites guttatus. PL XIV. fig. 2. 



Plaits broad and deep : outer plaits raised in large hollow bosses, 

 often elongated ; adjoining plaits having an occasional lateral 



