GASTEROPODS. 209 



ring in the Coal Measures. Some of the forms are widely dis- 

 tributed geographically, and a few have also a very consider- 

 able geologic range. A number of the now recognized species 

 will probably prove to be identical with forms previously known, 

 but these cannot be, with certainty, determined except by a 

 direct comparison of the type specimens. The synonymy, how- 

 ever, of a portion of the Macrochili has been made out as 

 indicated beyond. 



As already suggested, the forms of this group appear to 

 be easily separable into two sections the first typified by Mac- 

 rochilus acutum (Sowerby) and Soleniscus typicus Meek & 

 Worthen ; and the second having for its typical representative 

 Macrochilus ponderosum Swallow. The shells of the first 

 group are characterized by being more or less elongate or fusi- 

 form, with the spire elevated, acute ; body-whorl forming about 

 one-half the length of the shell ; aperture subelliptic, or oval, 

 acutely angular posteriorly ; columella imperforate, provided 

 with a conspicuous revolving fold or ridge, which, however, in 

 the perfect specimen is often scarcely discernible exteriorly, 

 but as it passes inward becomes more and more pronounced, 

 and is often accompanied by a second though much less promi- 

 dent fold of similar character ; test thick. 



The columellar ridge is in most examples usually hidden 

 more or less completely by the imbedding matrix filling the 

 aperture. By the removal of the outer lip the twisted fold 

 becomes more apparent. In a perfect specimen of Solenscus 

 neicberryi (Stevens), this ridge is scarcely defined at the aper- 

 ture, but toward the interior of the shell it gradually assumes 

 greater prominence, becoming very much elevated, very sharp, 

 and bordered on each side by a broad rounded canal, the outer 

 one of which is narrower and considerably deeper than the 

 other. On the inner margin of the second farrow there is often 

 developed an obtuse prominence, much less conspicuous than 

 the first, and best defined a short distance from the apertural 

 margin. From this point it soon becomes obsolete inwardly, 

 and finally disappears altogether. In the majorUv of the forms 

 G 15 



