OF CONCHOLOGY. 205 



unsymmetrical, the lower edge of the plate not being developed 

 on the side nearer the centrals, but thrown abruptly off towards 

 the margin of the membrane ; it is rounded and not trilobed like 

 the lower edge of the centrals ; the laterals are also longer, nar- 

 rower, with a less expanded upper edge of the plates than in the 

 centrals, in a contrary direction from which they are also thrown 

 off by the irregular curving of the plates. The cusps and denti- 

 cles on the laterals are the same as on the centrals. There are 

 no uncini. 



In one specimen were two abnormal rows along the whole 

 length of the membrane, in which the cutting edge was divided 

 into four instead of three lobes. These abnormal rows were 

 separated by a normal line. 



Fig. 4 shows one central and one lateral tooth. 



Helix orbiculata, Fer., (PI. 9, fig. 14.) 



Jaw striated, thick, short, broad, slightly arched ; ends 

 squarely truncated, scarcely attenuated ; cutting edge irregular, 

 but with no median projection. (See fig. 14.) 



Lingual membrane long and broad, composed of numerous 

 rows of about 47 — 1 — 47 teeth each. Centrals and laterals as 

 usual in the genus, the cusp being greatly produced. The 

 un(Mni are long and narrow, with a very greatly developed den- 

 ticle, much longer and quite as broad as the tooth, divided into 

 three lobes, of which the central is the largest. These uncini 

 resemble the laterals of Planorbis more than the usual form of 

 uncini in the genus Helix. The extreme uncini are more irregu- 

 larly denticulated, but the denticles still preserve the obtuse, 

 rounded form. 



Helix microdonta, Deshayes. 



The specimen examined is from Nassau, New Providence, 

 Bahamas. 



Jaw stout, long, broad, slightly arched ; ends but little attenu- 

 ated, bluntly rounded ; with ten stout, broad ribs, crowded into 

 the three inner fifths of the transverse diameter of the jaw, the 

 outer fifth at each end being free from ribs ; the ribs are visible 

 on both anterior and posterior surfaces, and crenulate either 

 margin. 



Lingual membrane long and narrow, composed of numerous 

 rows of about 20 — 1 — 20 teeth. Centrals and laterals as com- 

 mon in the genus, the plates subquadrate, the cusps rather short 

 and stout. Uncini long, narrow, irregularly denticulated, as in 

 figure of H. tridentata (L. and Fr. W. Sh. N. A., Part L, p. 

 130, fig. 220), but with shorter denticles. 



