834 THE EQUIANGULAR SPIRAL '[ch. 



constitutes the unduloid, lias not grown by successive increments 

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

 "rib" is often, perhaps generally, present on thef dorsal side of the 

 little conical shell. In a few cases (Limacina, Peraclis) the tube 

 becomes spirally coiled, in a normal equiangular 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 circular 

 or nearly so, becomes flattened or compressed dorsoventrally ; and 



3 -' 4 



Fig. 411. Shells of thecosome Pteropods (after Boas). (1) Cleodora cuspidata; 

 (2) Hyalaea trispinosa; (3) H. globulosa; (4) H. uncinata; (5) H. inflexa. 



the angle, or rather edge, where dorsal and ventral walls meet, 

 becomes more and more drawn out into a ridge or keel. Along the 

 free margin, both of the dorsal and the ventral portion of the shell, 

 growth proceeds with a regularly varying velocity, so that these 

 margins, or lips, of the shell become regularly curved or markedly 

 sinuous. At the same time, growth in a transverse direction pro- 

 ceeds with an acceleration which manifests itself in a curvature of 

 the sides, replacing the straight borders of the original cone. In 

 other words, the cross-section of the cone, or what we have been 

 calling the generating curve, increases its dimensions more rapidly 

 than its di^ance from the pole. 



