HELICOIDEA. Ill 



FAMILY HELICOIDEA. 



Shell heliciform, with usually depressed conical, or plane 

 spire, sometimes nautiloid ; or with depressed spire, generally 

 umbilicated, outer lip simple. 



Animal differing from the true Helices by the absence of the 

 jaw, and of the central tooth of the lingual ribbon. Carnivorous. 



There is but little to distinguish these mollusks either in the 

 shell or the external characters of the soft parts from some of 

 the groups of Helices, and the number of species of which the 

 mouth-parts have been examined is so small as to forbid any- 

 thing like an exhaustive list of the Testacelloid genera or 



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species ; I can only group around the known agnathous forms 

 such others as appear to be related to them by conchological 

 characters : time will show the real value of characters derived 

 from dentition and the presence or absence of a jaw. 



Synopsis of Genera. 

 Genus GUESTIERIA, Crosse, 1872. 



Shell imperforate, corneous, thin, depressed, quite involute, 

 the last enveloping whorl only visible, aperture semilunar, the 

 lip simple. South America. 



Genus DIPLOMPHALUS, Crosse and Fischer, 1872. 



Shell planorbiforin, openly umbilicated, spire depressed, aper- 

 ture narrow, lip simple. New Caledonia. 



{Typical.) Shell discoidal, showing the numerous narrow 

 whorls equally above and below, the extremities of the lip con- 

 nected by a much developed parietal callus. 



Subgenus MONOMPHALUS, Ancey, 1882. 



Shell nautiliform, spire somewhat depressed or nearly plane, 

 whorls less numerous than in the typical group, the last large, 

 almost covering the umbilicus. 



Subgenus PSEUDOMPHALUS, Ancey, 188*2. 



Shell nautiliform, with very narrow whorls and aperture, the 

 last whorl enveloping above and below, so as to exhibit a deep 

 sunken perspective spire and umbilicus. 



This form approaches Guestieria in appearance, and still more, 

 the European group Drepanostoma. 



