No. 490] 



ORTHOGENETIC VARIATION 



641 



angle develops a series of nodular spines, as the ribs gradually 

 become obsolete, slightly fainter nodules being formed where the 

 lower spirals and ribs cross. With the complete disappearance of 

 the ribs, the lower spirals — of which there are four large and three 

 smaller nioic closely ciowded ones in the adult — become con- 

 tinuous and free from iKHliiiations. No intercalated or secondary 

 spirals occur, except in accelerated types, where the shoulder and 

 nodules exist for only a few neanic whorls, after which they gradu- 

 ally disappear, only faint primary and secondary s|)irals remaining 

 on the last whorl (Mut. /? Fig. 9; C. U. coll. .lOOli',. In the type 

 specimen of M. praecessa figured by Deshnyes, the ribs have not 

 entirely disappeared in the adult. It represents a more primitive 

 or more retarded individual tlian the Xoyon \ariety described 

 above, its adult features being c()m])aral)le to the early neanic of 

 var. spiralis Grabau. Three vaiieties of .1/. iu'iuinafa (Def.) 

 are figured by Deshayes (C(K|. fo<s. Km. Paris II pi. 12), of 

 these var. a (Desh., figs. 3 and 11, our fig. 11) rs tlie nnniediate 

 successor of M. praecessa var. spiralis, but the tubercled character, 



the neanic and perhaps even in the nc|)ionii- whorls. \'ar. b 

 (Desh., figs. 15, 16) is characterized by the >up|)rcs.i(.n in the adult 

 of all the sj)irals e.vept the one on the angle, on which the tubercles 

 })ecom(> ureatly strengthened, and the spiral ju^T al.ovv xhc suture 



beomie strongly concave in the adult. The thinl'variety (c, figs. 

 7 and 8 of I)<'■^haye^ iia- all the tubercles suppres.sed, while the 

 spirals become >troni: and the whorls below the shoulder rounded 

 (Fig. 12). The |)liyietic relationships of the.se types may be 

 expressed as follows, the length of line representing relative amount 

 of divergence. 



var./. var. 11) var. c (Fig. 12). 



M. inquinaia -J 



I 



! Mut. /? 



M. praecessa var. spiralis ' 



(Fig. 10) 



