642 



THE AMERICAN NATURALIST 



[Vol. XLI 



M. praecessa var. spiralis 



M. praecessa 

 (Desh., An. sans Vert., II, pi. 23, figs. 31-32) 



Jurassic ribbed Pseudomelania 

 (D'Orbignv, Pal. Francaise) 



An interesting line of departure is shown in some siieciinens of 

 M. praecessa var. spiralis. Here the second sj)iral from the pos- 

 terior suture is the strong or primar}^ one; /. r.. ( arly in ihe ontog- 

 eny a new spiral appears above (posterior to) \\\v ])rimarv one. 

 This continues on the shoulder, and before the (lisapjx arance of 

 the ribs becomes somewhat tuberculose. Intercahated spirals also 

 appear on the aduh whorl. This appears to have been the lineal 

 ancestor of Melania asperaia Lam. var. /3 Brot, of the Philippine 

 Islands, a fresh water type (Figs. 14 and 15). The less accelerated 

 individuals of this form show the early ribbed whorls, though these 

 are marked by a large number of closely set extremely fine spirals 

 not observed in the Tertiary species. In the individual represented 

 by Fig. 15, a single row of spines makes its appearance after five 

 or six volutions. These spines appear near the center of the 

 exposed whorl, one on each rib, and begin shortly before the 

 appearance of the ribs themselves (Fig. 17). At first the spines 

 are very weak but gradually they increa.se in size, at the same 

 time beginning earlier and ending abruptly upon the rib. Together 

 the spines and their posterior prolongations produce the appearance 

 of spirals. The single row of spines continues for almost four 

 volutions the whorls grachially becoming divided into slionlder and 

 body with the ..pines on th<' pnuuMmced shoul.l.T angh'. A second 

 row appears on the shoulder beginning as a faint elevation which 

 is gradually >trengtlieiie.l. 



though the lower row becomes somewhat more strongly accentuated. 



