486 FREDERICK W. SARDESON 



the animal. The genus or subgenus Protwarthia (Bellerophon) 

 is also reported from the Calciferous. P. cassinensis Whitf. has 

 the shell more rapidly expanding, {i. c, shorter), the aperture 

 larger, and the sinus narrower by reason of its sides projecting 

 or building forward, as in the later Belleroplioji bilobatus (Fig. 29). 

 On the other hand, Oxydiscus, e. g., Bellerophon macer Bill., 

 while likewise bilaterally symmetrical, is convolute with abrupt 

 umbilicus, and the aperture cuts obliquely backward, forming a 

 wide sinus, the middle of which coincides with an angulation 

 which is a keel peripheral to the volution. 



A composite of these four genera and a transitional form 

 from P. siveeii to the other three would be such as the species 

 which I formerly described, as Raphistoma leisomellum Sar.^ 

 (Fig. 27), which consists of "about four rapidly increasing volu- 

 tions, which embrace in such a manner as to form a lenticular coil," 

 nearly equally convex above and below. The body whorl over- 

 laps notably the periphery of the spire, though it is not a plain 

 coil, the spire rising a little on the upper side, and an abrupt 

 umbilicus, about one-fourth the width of the shell, being on the 

 lower side. The periphery is narrowly rounded and slightly 

 inflated, and the aperture is like those in Bellerophon and Pleu- 

 rotomaria of the Calciferous. 



Contemporaneous with and related to P. sweeti are a number 

 of shells with high spires, in contrast to Bellerophon. They are 

 comprised in the name Murchisonia aiict., or again under several 

 generic terms — Lophospira, Clathrospira, Plethospira, Hormo- 

 toma, Coelocaulus, and Tunitoma. These have a sinus, the 

 abrupt middle of which produces a band upon the volutions, 

 often coinciding with a keel (Fig. 24) or one of the keels. None 

 of the species seem to have the sinus narrowed to a slit, as in the 

 case of some of the later genera. 



It may be true of these Murchisonia-like shells that the tran- 

 sition from a sinus to a slit has not yet been observed,^ and this 

 observation is not supplied here. The slit appears geologically 

 later in species which logically seem to be the descendants of 



'^Bulletin of the Mintiesota Academy of Natural Science, Vol IV (1S96), p. 99. 

 * Quarterly fournal of Geological Society, Vol. LVIII, p. 317. 



