188!*.] NATURAL SCIENCES OF PHILADELPHIA. 291 



that the type is somewhat deformed and is not a characteristic repre- 

 sentative of the species. 



Straparollus roberti (White). 



Euomphalus roberti White, 18(32. Jour. Boston Soc. Xat Hist., 

 vol. IX, p. 22. 



This species is very closely allied to S. lotus (Hall). It is, how- 

 ever, somewhat smaller, with the spire depressed below the upper 

 surface of the body whorl. The superior flattened area is inclined- 

 inwardly thus making the outer carina more prominent .than in 

 Hall's species. This form was described from the upper beds, and 

 occurs in the massive white limestone layers. It is, probably, the 

 genetic successor of S. lotus of the lower Burlington; and has 

 undergone but slight modifications in structure. 



There has always been a considerable diversity of opinion as to 

 the real relations of Euomphalus Sowerby and Straparollus Montfort. 

 The two groups have commonly been regarded as generically distinct, 

 hut equally good reasons have been advanced for considering them 

 synonymous, and some writers even go so far as to unite both with 

 Solarium Lamarck. Whatever may he the final decision in regard 

 to the genera established by Sowerby and Montfort, there appears, at 

 present, no reliable criteria by which a satisfactory separation can 

 be made of the planorbicular forms with angulated whorls and those 

 having the spire somewhat elevated and the volutions rounded. 

 Although the extreme representatives of the two are apparently 

 very distinct, the individuals are so variable and the gradations so 

 complete, even among those of the same species, that the generic 

 limits usually assigned are not tenable. Euomphalus is typified by 

 such forms as E. pentagonus Sowerby; Straparollus by S. dionyxii 

 Montfort. Among the Burlington series the first section is, perhaps, 

 best represented by Hall's Euomphalus latus, in which the depressed 

 -spire is nearly on a level with the body whorl. The upper surface 

 of the latter, being flattened, gives rise to two rather pronounced 

 angularities. The type of the second section has its best analogue 

 in S. macromphalus. But there are numerous transitionary forms 

 which are regularly rounded below and with only one carina above 

 or having the angularities very much rounded and in some instances 

 1 »;i rely perceptible. The spire, which is considerably elevated in some 

 species, in others becomes more and more depressed — even below the 

 level of the upper surface of the last volution. A number of other 

 generic names have been proposed which seem to be identical with 



