182 



Annals of the Carnegie Museum. 



Raphistoma immaturum (Billings). 

 Pleurotomaria immatura Billings, 1859, Canadian Naturalist - and 

 Geologist, Vol. IV, p. 454. 



Although this species has never before been figured, it may easily be 

 recognized from Billings' description, as it is the only species of 

 Raphistoma in the Chazy with step-like whorls. In Raphistoma 

 stamineum and Raphistoma striatum, when the central whorls are ele- 

 vated, the outer edge of the body whorl is also usually elevated so that 

 the outer whorl gives the whole shell a somewhat concave border. In 

 Raphistoma immaturum, each whorl is a little higher than the preceding, 

 making steps up to the apex (figure 2) and at the same time the top of 



each whorl slopes up toward the apex, 

 so that the surface is like that of Lio- 

 spiraamericana (Billings) of the Black 

 River. In all the specimens seen, the 

 lower part of Raphistoma immaturum 

 is less prolonged and ventricose than 

 in Raphistoma striatum and Raphis- 

 toma stamineum, and the whole shell 

 is more like the Black River and the 

 Trenton forms of Liospira. 



Description (Figure 2). 

 Shell small, consisting of five or six 

 whorls, each a little above the one 

 outside it. The apex is always ele- 

 vated and the slope is regular from 

 the apex to the outer margin of the 

 body whorl. The lower portion of 

 the body whorl is rounded, but is not 

 so conical as in Raphistoma stami- 

 neum. The angle at the margin is 

 acute. The section of the outer whorl 

 is almost a parallelogram, rather wide, gently convex in the outer lip 

 and base, and straight on the top. The umbilicus is closed in adult 

 testiferous shells, and small in young shells with test. 



The surface markings show numerous fine growth lines which bend 

 sharply backward from the suture, are interrupted by a raised line on 

 the middle of the top of the whorl, curve a little forward, then back 



Fig. 2. Raphistoma immaturum 

 Billings. Side and top views of an 

 imperfect specimen. Enlarged 4 

 diameters. 



