58 PROCEEDINGS OF THE ACADEMY OF [March, 



Whorls 6, 7, 8 — All characterized by the cancellated stage. Changes, 

 however, take place during these whorls. At first the ornamentation, 

 is much like that of the cancellated stage in V. limopsis, but later the 

 ribs become relatively farther apart and broader (in a spiral direction). 

 At the same time many fine riblets are introduced between the pri- 

 mary ribs. The tubercles are largest near the suture and become smaller 

 as the ribs die away anteriorly. There is never a distinct shoulder 

 angle. The whorls are covered with spirals. 



Whorl 9 — Individual variation now sets in. In some specimens the 

 whorl is much as in eight. In others the rib is swollen into a very 

 rounded, transversely long angle which hardly occupies the position 

 of the shoulder angle of later forms, and in addition the shells become 

 thick. In all individuals the mantle protrudes in the region of the 

 anal siphon, and tends to smooth the preceding whorl with a shelly 

 overgrowth. This latter is never extreme. 



Wliorl 10 — Much as in nine, though here the forms with swollen 

 ribs predominate. In many specimens the shells become thick, the 

 growth lines irregular, and the smoothing of the preceding whorl by 

 the shelly overgrowth is quite well marked, though it never becomes 

 extreme as in some forms of later time. 



Remarks. — This species in its earlier stages of growth closely re- 

 sembles V. limopsis. It differs radically, however, from that form 

 with the progress of its ontogeny. Though its individuals vary greatly, 

 it is nevertheless entitled (in the opinion of the author) to rank as a 

 distinct species. In its later whorls there is great individual variation, 

 accompanied by senile features. These latter, though never extreme, 

 are well marked. They comprise protrusion of the mantle in the 

 region of the anal siphon, producing a smoothing of the preceding 

 whorl by a shelly overgrowth; thickening of the shell, and irregularity 

 of growth lines. There is no spiny stage. V. rugatus is considered 

 to be a short senile offshoot from V. limopsis. In the absence of 

 intermediate forms this evolution must have taken place either at 

 some other locality or at some geologically earlier date. 



Volutilithes petrosus Conrad. 



The forms described at the present day under this name represent 

 an assemblage of races. If every gradation of evolutional develop- 

 ment did not exist among them, many of these races would doubtless 

 now be regarded as distinct species. In fact, there are several dis- 

 carded specific names which were used in this way by Conrad and Lea. 

 Each race is an assemblage of individuals, showing more or less in- 



