136 



PKOCEEDINGS OF THE MALACOLOGICAL SOCIETY, 



lateral area by a subangular ridge. Length of body-chamber not 

 seen ; aperture with a wide and deep hyponomic sinus. Chambers 

 not yery deep, measured at the centre of the periphery not quite one- 

 half of the corresponding height of the whorl in depth ; septa feebly 

 concave ; suture-line with a simple shallow curve on the lateral area 

 and an extremely shallow curve on the periphery. Siplmncle median. 

 Test rather thick, with distinct lines of growth, which are almost 

 straight on the sides and form a deep and wide sinus on the periphery ; 

 the inner area of the whorl, the inner portion of the lateral area, and 

 the periphery also with rather coarse spiral strige, those on the 

 periphery being rather obscure ; towards the aperture all these spiral 

 lines become obsolete. 

 Dimensions. 



Diameter 



Height of outer whorl 



Height of outer whorl above 



preceding whorl 

 Thickness 

 "Width of umbilicus 



Remarks. — Two forms of the species can be recognized. There is an 

 example of each in the British Museum Collection ; their dimensions 



(i) 



(ii) 



95 mm. 



80 mm 



50 „ 



44-5 „ 



37-5 ,, 



30 „ 



78 „ 



61 „ 



13-5 „ 



10-5 ,, 



Fig. XIII. — Nautilus semioniatus, n.sp. a, lateral view of one of the tj'pe- 

 specimens, an example of the more inflated form of the specieis ; b, front 

 view of the same specimen. Inferior Oolite : Bradford Ahbas, Dorset. 

 Drawn from an example in the British Museum CoUectiou [C. 4499]. 

 About three-fifths natural size. 



are given above. The broad form, with a flattened or slightly concave 

 periphery, is represented by the specimen No. C. 4499 (i\ and the 

 more compressed form, with a more convex periphery, by the example 

 No. C. 3244 (ii). The latter specimen is entirely septate, and shows 



