484 FREDERICK W. SARDESON 



which are known to us, though a few are probably from unknown 

 ancestors. In this Calciferous fauna there are, first, a number 

 of short obtuse cones, of which Tryblidhun ?iiobe Bill, and T. 

 nycieis Bill. (Fig 4) have the same general aspect as species of 

 of the Baraboo fauna; i. e., the shell smooth, curved with the 

 apex overhanging the narrower end of the ovate aperture. The 

 apex is, therefore, directed posteriorly, as I take it, though 

 authors generally have designated the same as the anterior, 

 because the line of paired scars of the type species were sup- 

 posed to be open at this end, in a similar way as the continuous 

 scar opens anteriorly in Patella. Ulrich and Scofield find other- 

 wise for Tryblidium, and it may be better compared to Archi- 

 nacella and Scenella, which have a continuous muscle scar which 

 is not open anteriorly. These two genera occur also in the 

 Calciferous zones. The one is low, smooth, conical, the other 

 somewhat higher and ornamented, the apex being either central, 

 or toward the broader, or toward the narrower end of the oval 

 aperture. They, with Metoptoma (Fig. 7), may readily be taken 

 to be the descendants of earlier species of Tryblidium, Hypselo- 

 conus, and Scenella, together comprising all such as have changed 

 to shortened or lower cones, and they may be taken as tending 

 toward the Patella, et al., of more recent ages. Further, an 

 interpretation of them might indicate that the apex is centrally 

 or posteriorly directed, except in the Hypseloconus line, which 

 would be the reverse of that, e:g., Metoptoma orithyia Bill. (Fig. 7), 

 and again that the broader end of the aperture is probably 

 anterior. 



Next to be mentioned is the genus Platyceras (Plate II, Fig. 

 15), which continues through and beyond the Calciferous zones, 

 and is still intermediate between Stenotheca and the dextrally 

 spiral shell type. Closest related to it is probably Holopea, but 

 this is not clearly derived from it. Strangely enough, the division 

 line between Diotocardia and Monotocardia, which so narrowly 

 separates Scenella and Stenotheca, would also seem to divide 

 the genus Holopea; or probably the Holopea-like species, when 

 better known, may be found to be distributed between Holopea 

 proper and Straparollus or a similar genus. 



