CREITHIID^. 121 



small, with tubular, short, anterior canal. Operculum paucispiral, 

 with subcentral nucleus. 



Animal with elongated foot, truncated in front, where it forms a 

 duplicature, the upper margin (mentum of Loven) less develoi)ed 

 than the lower ; tentacles long, cylindrical, united by a sinuated 

 veil, eyes at their exterior base ; snout very short ; operculigerous 

 lobe simple. 



Dentition (PI. 10, tig. 3). — Central tooth very short, transverse, 

 multicuspidate ; lateral tooth also transverse and multicuspid ; 

 marginals four, small, transverse, simple (4"l'ri'4). 



About a hundred living species are known, all quite small, in- 

 habiting the West Indies, Europe, Indian Ocean, Polynesia. Fossil. 

 Tertiary. Tristoma, Blainv., 1825, is a synonym. The Triforis 

 often attains a considerable size before losing its lan'al characters, 

 particularly when distant from the shore. Dr. Fischer has captured 

 on the high sea, between Provence and Corsica, a T. perversa, of 

 which the shell had eight or nine whorls, yet the animal still 

 retained its larval vellum. 



An anomaly of the shell is the occasional ^^reservation of a second 

 canal upon the back of the body whorl, showing the .termination of 

 a former aperture. This is present in the fossil species upon Avhich 

 Deshayes established his genus, and which is named from this character. 

 I)r. Jousseaume, in his recent monograph of the group, divides the 

 species according to the presence or absence of this posterior open- 

 ing, and creates certain new groups, of his division having three 

 apertures, which he admits are identical in other characters with 

 the sections Ino and 3fasto)iia of Hinds, which he places among 

 those having two apertures. Having examined several species with 

 three apertures I incline to the opinion that the posterior canal is 

 only accidentally preserved in some cases by reason of its deflection, 

 which causes its tube to be surrounded with shelly matter during 

 the growth of the shell, and that it bears no relation to the economy 

 of the animal. This view is fortified by the consideration that 

 neither in this group nor in any other group of the mollusca are we 

 acquainted with any oi'gan which might occupy or occasion this 

 tube for the purposes of its economy. Moreover, in one of the 

 species before me two individuals possess the third opening, whilst 

 two others have it not. In consequence of this view I relegate Dr. 

 Jousseaume's groups Iniforis and 3Iastoni(eforis to the synonymy of 

 Ino and Mastonia respectively. 



