XI] 



THE SHELLS OF PTEROPODS 



571 



increments or '"rings of growth," but has developed as a whole.) 

 A thickened "rib" is often, perhaps generally, present on the 

 dorsal side of the little conical shell. In a few cases (Limacina, 



Fig. 29(i. Diagrammatic tran.sverse sections, or outlines of the mouth, in certain 

 Pteropod shells : A, B, Cleodora austrdlis ; C. C. pymviirlalis ; D, C. bahintinni ; 

 E, C. rnsjjifldld. (After Boas.) 



Fig. 297. Shells of thecosome Pteropods (after Boas). (1) Cleodora 

 czispidata; (2) Hyalaea trispinosa: (3) H. globulosa : (4) H. uncinata ; 

 (5) //. inflexa. 



Peraclis) the tube becomes spirally coiled, in a normal logarithmic 

 spiral or helico-spiral. 



In certain cases (e.g. Cleodora, Hyalaea) the tube or cone is 

 curiously modified. In the first place, its cross-section, originally 



