1906.] NATURAL SCIENCES OF PHILADELPHIA. ^5 



not reached in all species, but occurs usually as the end term of normal 

 progressive development in the ontogeny, and of slow and even evolu- 

 tion in the phylogeny. 



The Spiny Stage, then, may sometimes be present as a well-defined 

 stage, but it may also be-accompanied by extreme individual variation 

 and by features of senility. These latter characterize the Senile 

 Stage, and may follow regularly after the Spiny Stage; but, as stated 

 above, they not infrequently are thrown back by acceleration, and 

 found together in the same whorl with the features of the Spiny Stage. 



The Senile Stage, whether following normally after the Spiny Stage 

 or associated with it, is always easily distinguished. In the forms 

 under consideration its most important features are the following : ^ 



1. Tendency for the shoulder spines to pass into a shoulder keel. 



2. Irregularity of growth lines. 



3. Thickening of the shell. 



4. Encroachment of the anal siphon upward on the preceding whorl. 



5. Protrusion of the mantle m the region of the anal siphon, pro- 

 ducing a smoothing of the preceding whorl by a shelly overgrowth. 



These five characters are all sometimes found together in one in- 

 dividual or race, but, as a rule, only two or three of them are so asso- 

 ciated. 



In studying the development of the shell features, each whorl has 

 been taken up in detail, and its particular ornamental characteristics 

 noted. Of course, this system is more or less arbitrary, and at times 



K3 



Fig. 1. — Line shows position taken for the end of the first whorl. 



inaccurate, on account of the acceleration of features in certain in- 

 dividuals. In the following study, the writer has endeavored to get 

 the average characters for a certain whorl, except where individual 

 variation is great. In this latter case the variations are noted. 



The word spine is therefore used, throughout this paper, simply in a descriptive 

 sense, and implies no difference from the early tubercle, except that it is larger and 

 sharper. Both are produced by the same fold in the mantle, and every gradation 

 between the two is observable." Smith, Burnett, Proc. Acad. Nat. Sci. Phila., 

 May, 1905, p. 347. This opinion is quite different from that held by Grabau 

 (see Am. Nat., Vols. 36, 37). 



^ For a discussion of senility see Smith, Burnett, Proc. Acad. N^at. Sci. Phila., 

 May, 1905. 



