478 FREDERICK W. SARD E SON 



long cone, or tubular. Coleoloides, one species, and Salterella, 

 three species, of short tubular shells, complete the list as given 

 by Walcott. 



The above species are very small shells, as compared to later 

 ones like those of the Ordovician, and they were the kind to be 

 the less easily and often fossilized because small and thin. There 

 is little doubt that their small size and simple structure are truly 

 representative of the earlier Cambrian marine Gastropoda of 

 the Canadian and New England region. 



The Baraboo fauna includes several species in three genera. 

 Tryblidium [Metoptoma Whitf.) (Plate I, Figs. 2, 3) includes 

 shells similar to Scenella, but the apex is more excentric and 

 directed toward the narrower end of the oval or acuminate 

 aperture, and the muscle scar is broken into distinct paired spots 

 arranged in a circle or in horseshoe-shape, open anteriorly. In 

 Hypseloconus (Plate I, Fig. 6) the apex rises posteriorly to 

 the center, but by the curvature of the shell is directed forward. 

 The muscle scars are apparently like those in Tryblidium. 

 Scaevogyra species (Plate I, Figs. 8, 9) are short, sinistral spiral 

 coils. Hyolithes primordialis Hall is the accompanying pteropod. 

 The western Upper Cambrian has therefore a primitive gastropod 

 fauna similar to that of the eastern Lower and Upper Cambrian. 



In interpreting these very simple types of gastropod shells 

 there is noticeable difficulty at the start in choosing the right point 

 of view. I have taken the generally oval aperture to be pos- 

 teriorly acuminate and the apex as directed backward. In 

 Scenella it ranges to anterior in position, but points rather 

 backward because of a concave slope on that side of the shell. 

 In Hypseloconus it rises posteriorly, but points forward. In 

 other cases the curvature or coil may be assumed as backward 

 directly or obliquely. There may have been a reason why the 

 aperture was posteriorly narrowed. The animal is presumed to 

 have been longer than wide, whence an oval shell could be well 

 fitted to it, and, in case the shell rested back of the middle, the 

 anterior of the shell might well be broader than the posterior, to 

 accommodate the body at retraction. The curvature may be 

 backward from some like simple cause. But in larger coils the 



