Plate IV. 



Deroceras planicosta. 

 Fig. 1. Section showing the imbricated structure of the median and exter- 

 nal layers of the shell. The lining layer is not represented. 

 Loc. Wiltshire. From the same block as the specimens of this 

 species previously figured, d", median layer of the shell ; d'", 

 external layer. Magnified 40 diameters. 



" 2. Section of shell transverse to the siphon at the juncture of the side 

 of one whorl with the abdomen of another, from same specimen 

 as PI. II, Fig. 2. d", median layer; d"', outer layer; d', lining 

 layer. Magnified 104 diameters. 



" 3. Enlarged portion from the abdominal side of same section. Magni- 

 fied 317 diameters. 



Nautilus Pompilius. 



Fig. 4. Section of young through the centre of embryonal umbilicus and 

 the siphonal coecum. d, lining layer of the shell ; d", median 

 layer ; d"', external layer ; d -}-, dark layer deposited by the 

 hood ; X, the plate which closes the apex of the whorl. Mag- 

 nified 15 diameters. 



Nautilus atratus. 

 Fig. 5. Side view of Saemann's original specimen, le, first septum. 

 " 6. Dorsal side of the same, showing the large dorsal lobe of the second 

 and succeeding septa. 



Nautilus Kontnckii. 

 Figs. 7,8,9. Ventral, dorsal, and side views, exhibiting three septa, and absence 

 of the rlutings on the apex of the young shell. 



Nautilus lineatus. 



Fig. 10. Section of the young. The external layer of the apex d'" is too 

 deeply shaded, and consequently seems to fuse with the dark 

 layer of the hood, which begins at d+. whereas it here becomes 

 thinner and passes inside of it, as in N. Pompilius. Fig. 4. 



GONIATITES LlSTERI, 



Fig. 11. Section of shell, which passes through the edge of a former aper- 

 ture at the end of the first whorl, exhibiting the imbricated struc- 

 ture of the layers. Magnified 40 diameters. 



Argonauta Argo. 

 Fig. 1 2. Longitudinal section of shell, but transverse to the fibres of the 



