PHYLOGENIC STAGE OF THE GASTROPODA 487 



those which have the sinus. The view of Ulrich and Schofield, 

 and Koken, that the sinus precedes the slit genetically is rather 

 unavoidable, since apparently in Bellerophon, Pleurotomaria, 

 et. al., in later geologic time a slit appears where in shells 

 of Calciferous age a sinus only occurs. There is, however, 

 formed in species of Calciferous age a more or less distinct band 

 by the more or less distinctly truncated bottom of the sinus. 

 The building out of the apertural margin so as to narrow the sinus 

 is in process of formation, though a "slit" would not be formed 

 until the sinus is narrowed to the width of the band. The slit 

 differs from the sinus in that growth-lines coincide upon its sides, 

 as they were tending to do in the sinus, and a slit appears to 

 have arisen from the sinus in several distinct genera, by parallel- 

 ism of development. Further, the theory may be extended to 

 the effect that the sinus, as seen in Protwarthia, Seeleya, Lopho- 

 spira, et al., is best explicable as the result of a genetically paral- 

 lel development, their common ancestor having then had only 

 its incipient stage developed. In the genera next to be men- 

 tioned here the presence of a sinus is not taken as evidence that 

 the ancestor common to them and the preceding genera had 

 other than a simple oblique shell aperture. 



The low coiled shells, Helicotoma and Ophileta, have, there- 

 fore, been well united^ with Straparollus in the family Enom- 

 phalidae. The Straparollus has, of course, the obliquely cut 

 aperture which I consider primitive. The other genera have the 

 outer lip produced, making a sinus which as a rule coincides 

 with a keel on the outer upper side of the whorls; and a band, 

 when present, runs upon the keel. A slit has not been per- 

 fected in any of them. Helicotoma (Fig. 19) has the apex of 

 the spire rather above the level of the elevated keel on the 

 upper side. Ophileta has it rather lower, and the shell is less 

 rapidly expanding than in Helicotoma. Another genus similar 

 to these, Ecculyomphalus (Fig. 18), is coiled in one or more 

 free volutions in nearly one plane. Authors generally have con- 

 sidered it an uncoiled shell, though it is better to regard it as 

 coiling, showing a comparatively less rapidly developing retarda- 



' Ulrich and Schofield, Geological Survey of Minnesota, Vol. Ill, p. 1023. 



