01RTY, INVERTEBRATE FOSSILS FROM COLORADO 7 



and standing at approximately a right angle to one another. The lower is 

 gently concave, more so than the upper, although the upper spoons outward as 

 it approaches the carina. Suture deeply depressed. 



The most conspicuous superficial feature consists of narrow angular costse, 

 leaving between them broad shallow interspaces, which cross the upper portion 

 of the volutions transversely or in a direction longitudinal to the shell as a 

 whole. They are straight, but are slightly oblique, retrally directed from 

 above downward. These plications are perhaps restricted to the three or four 

 older volutions, and there is some irregularity in their arrangement. They 

 die down before reaching the carina. In addition, the whole surface of the 

 upper zone is marked by microscopic transverse and revolving lirae producing a 

 more or less cancellated effect. The revolving lirae are rounded, closely 

 arranged and prone to be wavy. The transverse lirae are finer, sharper and 

 more irregular, more of the nature of incremental lamellae, and the costse may 

 perhaps be looked on as fascicles of these markings. The lower zone of the 

 volution is marked similarly to the upper, but the angular costse are less strong. 

 They have a slight forward obliquity from the carina. There appear to be two, 

 possibly more, strong rounded revolving line on the final volution at a point, 

 as it would appear, about half-way down from the carina, and the volutions so 

 embrace as to leave about two of these lirse visible above the deeply sunk 

 suture. The final volution is not well shown by the specimens examined, so 

 that the sculpture below these two lira?, the relative distance at which they 

 occur below the carina, the shape of the aperture, etc., are not known. The 

 carina is the site of the slit band. The band is occupied by two rather coarse, 

 rounded lira?, separated by a narrow stria and appears to be defined by two 

 delicate lamellose lines, one above and one below, bounded on the median side 

 by slight striae. The two revolving lirae which occupy the whole of the band 

 and are more projecting than the edges are rendered nodose by the costae 

 described as crossing the upper and lower surfaces of the volution. That is, 

 the swellings occur where the costae would cross them, but the costae are evan- 

 escent on the upper surface near the band, and the nodes are much more 

 prominent than the costae and much more elongated spirally. 



In its specific relations, this shell is most nearly related to Marcliisonia 

 lasallensis and M. terebra. It differs from both in the presence of trans- 

 verse plications. Prom terebra:, which seems to be more nearly related 

 than the other, it apparently differs also in having the carina containing 

 two crenulated lirae instead of one, in having two revolving lirae just 

 above the suture and in other details of sculpture. 



Generically, this shell can hardly be classed with typical Murchisonia, 

 though it belongs to a group frequently cited under that genus. In some 

 important respects, it is comparable with such representatives of the 

 genus Worthenia as W. tabulata. This is especially true of the structure 

 of the slit band, which seems to be identical in both. Given a much 

 higher spire and more gradually enlarging volutions, with some modifica- 

 tions in the modeling of the whorls, especially the lower part, it is easy 



