No. 490] 



ORTHOGENETIC VA RIA TION 



615 



tubercular keel, and spines. Of these, so far as ascertained, the 

 ribs generally appear first, though in a group of early gastropods 

 in which ribs are never developed in the adult, spirals may be 

 well developed. The point of importance, however, is not the 

 relative time of appearance, but the mode of development of each, 

 and their influence upon each other when they occur together. 

 Where the development is complete, the ribs are . at first faint 

 vertical ridges upon a rounded whorl, enlarging gradually until 

 they are strong, rounded, elevated ridges extending from suture to 

 suture, with their greatest prominence at the ambitus of the 

 whorl (Figs. 16-17). From being at first ratlier distant, they may 

 increase in breadth until they are separated only In- a depressed 

 line. With the appearance of the anibital angulation, the ribs 

 become fainter towards the sutures, while at the same time they 

 become more pronounced upon the ambitus. Eventually the ribs 

 disappear entirely from the shoulders, disappearing at the same 

 time or somewhat later from the body of the whorl below the shoul- 

 der angle. Then only a regular row of tubercles remains behind, 

 each tubercle being the concentrated essence of the ribs of the 

 earlier stages (Studies II, fig. 10). Throughout, this process of 

 metamorphosis is a regular progression, though in some fi. e., 

 accelerated) shells it occupies a smaller number of wliorls tliaii in 

 others. Up to this point there is uniformity in development of all 

 ribbed shells with angular whorls following upon rounded ones. 

 Beyond this point there is a divergence; along one line the tubercles 

 gradually broaden and become confluent into a keel (Studies II, 

 fig. 12) which forms a prominent and persistent character, dis- 

 appearing only in old age types when the shell becomes rounded; 

 along another line the keel, if it comes into existence quickly dis- 

 appears, when the shell becomes rounded in outline. Upon this 

 a new type of ornamentation, the spine, a])pears. This will be 

 more fully discussed below. 



The spirals.— These are continuous arul persistent folds in the 

 shell substance, caused by a slight fnuirgination in the lip of the 



mantle when withdrawn, the growth of that organ being a more 

 rapid one thnn is conHucnsurate with the increase in diameter of 

 the whorl. I'hus while when expanded, the mantle is smooth. 



