4 ARKIV FÖR ZOOLOGI. BAND 8. NIO 25. 



the fossil specimens Kved in a more arctic or a more temperate 

 climate than that prevaling nowadays in Bel Sound. The 

 presence of Mytilus in the same layer might perhaps indicate 

 a somewhat warmer sea, but the fact of the presence alone 

 of one species must be used with caution when conclusions as 

 to the climate are to be drawn; of ten it is only a more de- 

 tailed examination of the stage of development attained and 

 of the variation of the fauna or some characteristic portions 

 of it that can give us certain criteria for judging in such 

 problems. The small or young forms present in the Ptisanula- 

 bearing layer were not suitable for a comparison with recent 

 ones, but perhaps it might be possible, from further collec- 

 tions, difinitely to solve this question. 



Description of the Radula (Figs. 6 — 8). The formula 

 of the radula is 1 . 1 . 1. It contains 26 complete and 3 rudi- 

 mentary series of teeth. The median tooth is strong and 

 high, the sides of its base are produced in wing-like processes 

 with a deep sinus ' between them; a short collum separates 

 the basal portion from the apex, which itself is a little nar- 

 rower than the base and is spoon-formed with the concave 

 surface to the front and a slight concavity behind; its margin 

 is erect and serrated into 7—9 strong denticles. The median 

 teeth are placed close together and in articulation with each 

 other (Fig. 8). The lateral teeth are lamelliform (Fig. 8) and 

 very thin; they are placed with their basal edges in the 

 longitudinal direction, and thus they cover the median teeth 

 (Fig. 6) and have a concave outer surface. They have the 

 shape of a meniscus with a strongly convex anterior margin 

 and a less sinuous posterior one. Their upper ends rise 

 beyond the median apices (Fig. 6) and are deflected laterally 

 owing to the concaveness of their surface (Fig. 7). The 

 median teeth have a dark brownish colour, that of the lateral 

 ones being lighter. 



The Animal is capable of withdrawing completely into 

 the shell (Fig. 3). In the retired position the foot-sole is 

 expanded, not folded, and fills the aperture like that of a 

 Limncea, owing to the fact that it lacks an operculum. The 

 foot has a broadly oval form with an unbroken regularly 

 rounded, and tapering margin all around. In the median 

 line of the sole there runs a longitudinal furrow. On trans- 

 versal sections this is seen to protrude very deeply in the 



