FOSSILS OF THE CLINTON GROUP. 553 



beneath, though from its position each whorl hides a little of the previ- 

 ous one. Tjie surface is marked by numerous close, fine, transverse 

 striae, too fine as a rule to be seen without a lense. Traces of very fine, 

 distant, longitudinal striae are indistinctly visible under an ordinary 

 lense; in other cases they might be altogether obsolete. No longitudinal 

 carinae of any kind are visible. 



This species was found in the "Orthoceras block" at Huffman's 

 Quarry, Ohio. 



Platyceras {Platy stoma) Niagarense, Hall. 



(Plate 25, Figs. 3 a, b; 22 a, b.) 



This is in some respects quite a variable species, yet on close inspec- 

 tion it will be seen that the variations are characteristic chiefly of the 

 later stages of life, and hence are usually found in the last whorls or at 

 least the later ones, while the earlier whorls of various forms are apt to 

 be closely similar. Thus the earlier whorls of almost all specimens form a 

 quite regular, though rapidly widening spire, in which smaller whorls 

 successively expose about half of their height above the larger, partly 

 enveloping, succeeding whorls. At this stage the aperture shows a dis- 

 tinct though neither large nor deep umbilicus, a thin outer lip, and an 

 inner lip, the latter, after having passed the umbilicus, apparently termin- 

 ating on the lower surface of the previous whorl — so that the upper end of 

 the outer lip (holding the shell with the initial end up) appears to join the 

 previous whorl independent of the lower lip. In reality, the upper inner 

 edge of the lip has grown tight to the previous whorl. 



From these similar younger stages of growth are developed the differ- 

 ent variations. The normal and mo.e common type shows a very rapid 

 expansion of the last whorl, which, however, does not envelope the pre- 

 vious whorl much more than is the case with the younger whorls, so that 

 a moderately elevated spire is produced. L,ess commonly this last whorl 

 envelopes the previous whorl to a greater extent, at least for two-thirds 

 its height, thus giving the upper, apical part of the shell a much more 

 flattened aspect. In these forms it is usual to find the upper part of 

 the inner lip still adnate to the surface of the previous whorl, even in 

 specimens 20 mm. wide (Plate 25, Fig. 22). There is also a much rarer 

 variety, comparable to the variety plebeium, Hall, in which the last whorl 

 leaves from two-thirds to four-fifths of the height of the previous whorl 

 exposed, thus producing a much more elevated spire (Plate 25, Fig. 3 b). 

 These forms all show, with very rare exceptions, well marked though 

 fine striae of growth; fine revolving striae. are also present; they often 

 equal the transverse striae in distinctness, though quite as commonly they 

 are less distinct. Very rarely the revolving striae are very distinct and 

 the transverse are almost obsolete. These various forms occur at Soldiers' 

 Home, Fauver's, Centerville, Fair Haven, and Brown's Quarry, Ohio. 



