STREPTAXID^E. 59 



summit, whorls numerous, flattened, pale corneous or whitish, 

 usually ribbed-striate ; aperture oblong or subtetragonal, with 

 or without teeth, lip with reflected margin, the extremities 

 united by a callus, columella almost straight, unarmed. 



I have adopted for Gibbus, the extended limits assigned it by 

 Dr. Paul Fischer, including Ennea as a subgenus. Whilst there 

 are great differences as to size, and some in form, between 

 species of Gibbulina, and the usual manifestation of Ennea, 

 still there exist some intermediate species ; and the differences 

 between these two typical groups are certainly no greater than 

 those of the two principal sections of Streptaxis. Montfort's 

 name, being the oldest, is adopted for the entire group, instead 

 of the restricted use of it by its author, followed by Albers and 

 Pfeiffer, for the aberrant form which is its type. 



The geographical distribution of Gibbus is much more re- 

 stricted than that of Streptaxis, being confined (except for a 

 single introduced species) to the old world. The larger and 

 more typical groups occur in Mauritius, the Isle of Bourbon, 

 Madagascar and the Seychelles Islands ; being entirely insular: 

 Ennea, on the contrary, is essentially (but not entirely) conti- 

 nental, being largely tropical African and Asiatic. Together, 

 fully one hundred species have been described. 



Section GIBBUS, Montf., 1810. Shell subconical, whorls flat- 

 tened on the front and back. G. Lyonetianus. 



? Section PASSAMAELLA, Pfeiffer, 1877. Subconical, obliquely 

 regularly striate, last whorl dilated below at the base of the 

 aperture, widel} r umbilicated; aperture coarctate, ringent, den- 

 tate. G. Passamainus, Petit. Island of Socotra. The position 

 of this species is extremely problematical : it may belong to the 

 Pupae. 



Section GONIODOMUS, Swainson, 1840. Shell ovate, whorls 

 ventricose, obliquely costulate, aperture directed obliquely 

 outwards at its base. G. pagoda, Fer. 



There is a slight displacement of the whorls in this single 

 species (to which the group is restricted), which appears to 

 connect Gibbus with the following group. 



Section PLICADOMUS, Swainson. 1840. Shell ovately subconic, 

 apex obtuse, whorls regular, surface obliquely costulate. G. 

 sulcatus, Mull. 



