No. 490] ORTHOGENETIC VARIATION 



645 



corvenii Fer.) of the same locality. This begins with a round- 

 whorled ribbed stage passing into an angulated stage in which the 

 spirals are prominent. The ribs progressively break up into tuber- 

 cles, those on the median spiral being moi'e prominent. In acceler- 

 ated individuals (P. radula Linn.) the upper of the two median 

 spirals has its tubercles slightly more accentuated than the lower, 

 while a secondary spiral between them is also slightly tuberculated. 

 The young of P. fmcatm Linn, shows the bicarinate whorls, the 

 spirals forming the two carinae being tubercled. Somewhat later 

 the upper series of tubercles becomes accentuated and a second- 

 ary spiral appears between the two. Up to this point, perhaps one 

 fourth the length of the shell or less, all the stages except the 

 earliest of P. granulatm- are repeated, so that the young of P. 

 jiiscatus is in effect a diminutive P. gramdatus. The further 

 development of P. juscatiis is along the line of accentuation of the 

 tubercles of the upper spiral, until they have become pronounced, 

 sharp, and spine-like in the adult, all the other spirals, except the 

 one just alx)ve the suture, disappearing in the most specialized 

 examples. The series in this case is as complete as is that of the 

 spinose Clavigers, and representative species, showing the same 

 stage of development so far as the surface characters are concerned 

 can be selected in both series. 



The carinated ]Melanias represented by Claviger matoni Gray 

 (Martini Chemnitz pi. 37; 3, 3a-f; 4, 4a-6) from Senegal show 

 another interesting type of modification. The primitive species 

 (C. midans Gould, Martini Chemnitz pi. 37; 3b-e) are spiralled 

 and ribbed, the upper spiral being the stronger. In somewhat 

 more accelerated individuals the ribs are lost on the last whorl, 

 but the spirals continue and become stronger, especially the upper 

 one which begins to extend outward as a strong flange form- 

 ing a deep notch or sinus in the lip. Somewhat more acceler- 

 ated specimens show tsvo or three final whorls ni this condition, 

 the flange of the last one becoming extreiiK-iy y)ron<)un('e(l. while one 

 or in case of more loosfly (oiling- individuals. i\v(M.f tlir lower sjnrals 

 also begin to be strengthened. Finally in e\treinely accelerated 

 specimens, the ribbed and spiraled portion is restric-ted to the 

 apical part, and it passes almost abruptly into a smooth shell on 

 which the upper spiral makes a pronounced flange-like carina. 



