58 STREPTAXID^E. 



Subsection AMMONOCERAS, Pfeiffer. Shell depressed, or- 

 bicular, shining, pellucid, thin, radiately striate ; umbilicus 

 wide, perspective ; spire rather flattened ; last whorl not 

 descending at the aperture, the periphery rounded ; aper- 

 ture large, rounded lunate ; lip simple, acute, its extremities 

 approaching. 



With this group may be united Scolodonta, Doring. 

 Section DISCAKTEMON, Pfeiffer. Shell discoidal or depressed 

 conoidal, volutions regular ; openly umbilicated ; aperture trans- 

 verse, somewhat basal, outer lip reflected or expanded, thickened, 

 and together with the parietal wall, usually armed with teeth. 



South America. 



Section EUSTREPTAXIS, Pfr. Axis of the shell oblique, causing 

 some whorls to revolve obliquely to the others. 

 Subsection 1. Edentulse. 

 Subsection 2. Dente parietali munitae. 

 Subsection 3. Pariete aperturali et peristomate armatis 



(Odontartemon, Pfr.). 



The prolonged lamella of the parietal wall in S. Troberii is 

 thought, by Mr. Ancey, sufficiently distinctive for the formation 

 of a new section, Lamelliger ; but in some of the other species 

 there are intermediates between this kind of tooth and the more 

 usual short tubercle, so that if the group were adopted it would 

 be difficult to assign all the species. At first I was disposed to 

 regard the claim of Oophana to recognition as a distinct group 

 as more valid, but a comparison on my plates of the typical 

 forms of L. bulbulus and S. Micliaui with S. turbinatus, S. 

 Mozambicensis, S. decipiens, S. obtusus, S. Enneoides, S. Wel- 

 witschi, etc., shows that the former only form a part of a con- 

 tinuous series by which the usually prostrate Asiatic species are 

 connected with Ennea. 



Genus OMPHALOPTYX, Bottger, 1875. 

 Fossil only. See " Structural and Syst. Conchology," iii, 16. 



Genus GIBBUS, Montfort, 1810. 



Animal like Streptaxis ; the teeth are arranged in very oblique 



rows, the central tooth elongated, subirregular, simple, unicuspid. 



Shell rimate or perforated, pupiform, oblique, with obtuse 



