28 



commence to descend towiirds the sutures; the })cri])hery is rounded 

 but slig'htly tlattened. The ap(;rture is suliovate, its base being con- 



FiG. 3. 

 Fif4. 3. — A. LoGANiANUS, outline of the aperture of a typical specimen. 



cavely eniarginate. If measured in the centre, Avhere the emargination 

 is greatest, the height of the aperture is rather less than its width, 

 but as viewed externally the height and width of the whorls are nearly 

 equal. 



The umbilicus of the most perfect s[)ecimoa is rather less than one- 

 third of the entire' diameter; its inner face is somewhat straight and 

 preciijitous below ; its outer margin is evenly rounded. It is deeply 

 excavated in the centre of the shell, but becomes shallower very rapidly 

 towards the mouth. 



The surfiice oi-namentation consists of primary, trifurcating ribs, which 

 usuallj" alternate with secondar}', simple and shorter ones. Commencing 

 at the sutures, the jji-imaries are at tirst distant, obtuse and prominent; 

 then at about a third of the distance across the sides, they trifurcate and 

 pass over the perij)hery, reuniting at exactly similar points. The points 

 where the primaries begin to trifurcate are marked by small elevations, 

 or tubercles.* The intervening costa^ are simple and do not extend to 

 the sutures, but become obsolete near the middle of the sides. Some- 

 times the secondary ribs are absent, and there is rarely more than one 

 between each jjair of primaries. Although much worn in the actual 

 specimens, there is reason to suppose that all the costa3 were originally 

 acute ; the grooves between them are concave, and a little wider than the 



* In one specimen, these are narrow and elongated ; in the other, whose surface is much abraded, 

 they are rounded and obtuse ; in both they are feebly marked and inconspicuous. 



