PHYLOGENIC STAGE OF THE GASTROPODA 485 



The Calciferous species of Holopea (Plate II, Fig. 21), 

 Trochonema (Fig. 22), Straparollus, if the presence of this 

 genus is conceded, and Raphistoma (Fig. 26), including Raphisto- 

 mi?ia, U. & S., are advanced little, except in respect to size, 

 over earlier Cambrian dextral shells. They have the aperture 

 simply formed, i. e., cut obliquely from the sutural side (posterior) 

 to the opposite (anterior) side. Holopea has rounded whorls, 

 slightly impressed, forming a moderate spire with open umbilicus. 

 Trochonema is similar, with angulations of the right or upper 

 side of the aperture, producing keels on the whorls. Straparol- 

 lus is a longer tube, coiled with wider umbilicus and more nearly 

 discoid than Holopea. Raphistoma is low-coiled, with angulated 

 anterior or periphery. 



Other and associated types in the main may be viewed as 

 modifications of these, by change of aperture and spire. In the 

 case of these — to make a theory to fit the phenomena — there is 

 a tendency in many species toward an extension of the apertural 

 margin at the under outer side of the whorl, which produces a 

 sinus, as in Pleuroto?naria {Seeleya) sweeti Whitf.^ (Fig- 23), 

 which otherwise is like Holopea (Fig. 21.) Whether this aper- 

 tural change is considered to be the building forward of certain 

 parts or the cutting back of another, as writers generally have 

 held of such shells, the species are in later time followed by 

 others in which the change is more developed, the Calciferous 

 species appearing to be midway in an evolutionary change which, 

 in general terms, is a tendency to produce the aperture nearly in 

 the plane of the shell's axis of coiling, /. e., transverse to the 

 volution. At the same time a notch is formed. The sinus in 

 this case arises apparently not in front, but to the right side. 



Closely related, though a link nearer to it may be shown, is 

 the Bellerophoji {^Owenella) mitiqjmius Whiii. (Fig. 28), which 

 is bilaterally symmetrical, since the spire is drawn in or sunken 

 to the median plane of the volutions, and at the same time the 

 aperture is built out below symmetrically, to form a broad sinus, 

 which probably in this case was shifted to anterior, median of 



' Holopea sweeti Whitf. There can be no doubt that the specimens in hand are 

 Whitfield's species. 



