G. C. Crick — Note on Actinocainax, etc. 



409' 



Some of the forms that are referred to the genus Actinocamax do 

 not exhibit the convex alveolar end referred to by Miller; a short 

 explanation may therefore perhaps be of interest. 



The form of the alveolar end of the guard in this genus depends 

 upon the forward extent of the calcification of the component layers 

 of the guard. All the species of Actinocamax possessed phragmocones . 

 In the typical form A. verus the alveolar end is convex or more or 

 less conical, the apex of the cone being sometimes occupied by 

 a minute rounded shallow depression, indicating the position of the 

 protoconch or commencement of the phragmocone. During the 

 growth of the guard the alveolar portion of each component layer 

 remained uncalcified, the extent of the uncalcified portion increasing 

 with each successive layer (see Fig. a). Consequently, when 

 during fossilization the uncalcified portion decayed and was lost, 

 the phragmocone became detached, and the alveolar end of the 

 guard assumed a convex or pyramidal form. 



cc 



Diagram of longitudiual sections of different forms of Aclinocama.v. The dotted 

 lines indicate the uncalcified part of the guard that is lost during fossilization. 

 a, typical form, A. verus ; h, A. granulatus ; c, A. qnadratus. 



If during the growth of the guard the alveolar portion of each 

 component layer remained uncalcified, whilst the extent of the 

 uncalcified portion gradually increased with each successive layer, 

 but much less rapidly than in the typical form, then the fossilized 

 portion of the guard would have a conical hollow alveolar end 

 (Fig. 6) having an angle greater than that of the alveolus or of 

 the phragmocone. Such a form is A. granulatus. 



Again, if the earlier (inner) layers of the guard were completely 

 calcified, and the rest remained uncalcified in the neighbourhood 



