TURBINID^E. 163 



at the base of the cusp. (PI. 61, fig. 1.) It is desirable that ad- 

 ditional specimens be examined. 



Tricolia, The type species, P. speciosa, has a broad simple median 

 tooth, overlying the bases of the inner laterals ; these are sub-rhom- 

 boidal, produced at their outer angles into wings which overlie the 

 bases of the adjacent tooth outward, and have denticulate cusps. 

 The outer lateral is narrow, not produced on the outer angle. The 

 marginal teeth have long simple cusps, (pi. 60, fig. 72.) Troschel 

 has figured the radulse of P. pulla, (pi. 61, fig. 2.) P. Kochu and 

 P. capensis, all agreeing with P. speciosa.. The following also I have 

 found to be of the same type; P. tennis, P.umbilicata, P. compta, 

 P. affinis, P. fordiana. The jaws of the latter species are figured 

 (pi. 60, fig. 69). Several of these species lack the outer lateral tooth, 

 the formula being oo* 4' 1. '4' oo . 



Orthomesiis. In this group the median tooth is reduced to a lin- 

 ear rudiment or wholly absent. The lateral teeth are of a squareish 

 form, their outer angles scarcely or not at all " winged," and but 

 slightly or not at all projecting over the adjacent tooth. They form 

 straight rows across the middle of the membrane, not V-shaped, as 

 in the preceding groups. In the typical species, P. variegata, the 

 laterals bear long cusps, with minute spurs at their bases ; the inner 

 marginals are very large, with enormous cusps, the outer ones with 

 long denticulate or laciniate cusps (pi. 61, figs. 3, 4.) Troschel fig- 

 ures this type of dentition for P. variegata, P. flammulata, Phil, and 

 P. lineolata. I have found the same in P. nivosa Rve., (=P. varie- 

 gata Lam.). In P. virgo Angas (pi. 60, fig. 70) I have found an ex- 

 tremely peculiar and interesting modification of the Orthomesm type 

 of dentition. The two inner lateral teeth are united by their inner 

 edges, forming a pseudo-median tooth ; the free laterals remaining 

 are three in number on either side, and are of the same form as the 

 one shown in the figure. As in the smaller species of Tricolia, the 

 narrow outer (fifth) lateral has evidently been lost. The marginals 

 all have long laciniate cusps. The lateral teeth have the basal plate 

 projecting at the upper outer angle, forming a sort of hood, which 

 reminds one. of a similar structure in the median teeth of Littorina. 

 This " hood," marked a in the figures, is probably homologous with 

 the similarly marked basal plate which projects above the cusps in 

 P. variegata, etc. The reduction in number of the teeth in this form 

 is probably due to the same cause which has, I believe, acted in the 

 mure minute members of the Pulmonata and other groups which 



