PHYLOGENIC STAGE OF THE GASTROPODA 4// 



primitive or whether advanced in Cambrian time. Walcott^ 

 describes about twelve species of gastropods in the Olenellus 

 zone, or Lower Cambrian, and as many of the Hyolithoidea, or 

 supposed pteropods. The Gastropoda include four species of 

 Scenella (Plate I, Fig. i) which are conical shells, about as high 

 as wide ; aperture elliptical or oval, tending to narrow posteriorly 

 when the apex also inclines backward. The species are rather 

 variable as described {loc. cit.^, and in that respect not unlike 

 the species of the Ordovician.^ The genus apparently merges 

 into the Patella of Silurian to Recent time, and is therefore of 

 the Cyclobranchia as now known. Stenotheca (Plate II, Fig. 

 13) includes three species, distinguished from Scenella by con- 

 centric wrinkling of shell. A tendency in such shells toward 

 curvature of the apex is worthy of note (5. curvirostra S. and F.) . 

 Platyceras, two species, includes P. primaevum Bill (Plate II, 

 Fig. 14), which has a short, dextrally coiled apex or spire. 

 The apex coils obliquely to the assumed dorsal direction. The 

 aperture has the narrower side next the suture. Straparollina 

 remota Bill (Plate II, Fig. 20) is a short dextral coil, with spire 

 depressed and rounded in outline; whorls nearly uniformly 

 rounded, more narrowly so on the upper side near the suture 

 and also on the basal side. Raphistoma attleboroughense S. and F. 

 (Plate II, Fig. 25) is a small dextral shell of three spiral whorls. 

 These are rapidly expanding, closely coiled, and rather narrowly 

 rounded on the outer side, which is a little thickened. 



Helenia bellaWdXc. (Plate II, Fig. 17) is long, curved dorsally, 

 and has an elliptical aperture, beveled toward the posterior. The 

 apical end of it is imperfectly known. Similar to this one are 

 the genera supposed to be pteropods. Hyolithes, including 

 eight species, are long, conical, transversely broadened, anteri- 

 orly flattened, dorsally more or less angular, and the aperture 

 is extended at the anterior side. There are sometimes apical 

 loculi, and there is anoperculum. In some there is a tendency 

 to curve dorsally. Hyolithellus includes one species — a very 



' U. S. Geological Survey, Tenth Atinual Report, pp. 616-25. 



*The variability of Ordovician species of Scenella, Archinacella, etc., has been 

 greatly underestimated by some authors. 



