148 SYSTEMATIC ARRANGEMENT 



§§ Axis of shell imperforated, conically concave in front. 



3. Trochus. 

 Pillar-lip twisted, simple; axial cavity smooth; throat smooth. 



* Axial cavity moderate, smooth, conical. 



1. T. niloticus, t. 43. f. 1. 



** Axial cavity very deep, conical. 



2. T. asper. 



*** Axial cavity very deep and wide, open. Infundibulum. 



3. T. concavus. 



4. Rochia. 



Pillar-lip twisted, simple ; axial cavity deep, narrow, smooth, with 

 a strong fold on the pillar-lip, and a deep sinus behind, near the whorl. 



1. R. acutangula. 



5. Prjecia. 



Pillar-lip twisted, simple ; axial cavity deep, narrow, with a 

 distinct narrow, central, spiral rib ; throat striated. 



1 . P. elegantula. Trochus e., Gray. 



6. Anthora. 



Pillar-lip twisted, simple ; axial cavity moderate, narrow, with 

 several opake subspiral ridges. 



1. A. tuberculata. Polydonta t., Gray. 



7. Polydonta. 

 Pillar-lip two- or three- twisted ; throat sulcated. 



* Axial cavity smooth or slightly ridged. 



1. P.? tiaratum, t. 64. f. 6, 11. 



2. P. sandwichense, t. 107 a. f. 5. Frontal appendage distinct, 



lobed ! 



** Axial cavity with regular spiral ridges. 



3. P. Hanleyanum. 



Lingual membrane of Polydonta umbilicatum linear elongate, trans- 

 parent. Teeth 00 * 5 * 1 • 5 • 00 ; central tooth small, laminar, longer 

 than broad, rather contracted above, square below ; apex rounded, 

 slightly recurved; inner lateral teeth 5*5, similar, equal, linear, 

 rather oblique, with a rather large, thickened, rounded, slightly 

 recurved apex ; outer lateral teeth very numerous, hair-like, linear, 

 equal, similar, with the apex recurved. 



