168 MOLLUSCA. 
In its fawn-colour, faint sculpture, and simple parietal plait, this species approaches 
very near the subgenus Triodopsis, and might equally well be placed in it. 
It varies much in size: the largest example I have seen is from Mexico city, and it 
measures 10 millim. in diameter and 7 in height; the smallest is an excessively little 
specimen from Sayula, 74 millim. in diameter and 4 in height. This Sayula shell has 
half a whorl less than the other specimens from the same locality (which are 9 millim. 
in diameter), but the aperture is distinctly bent down, and the thickening of the 
peristome, the plait, and teeth are present, though somewhat thin; these parts may 
have become thicker if the animal had lived longer, but they could scarcely have 
sensibly increased in diameter, because the aperture is essentially formed already; it 
may be called a premature specimen. By the separation of an accidental fracture 
during life the aperture can be somewhat deformed, even so as to show a prominent 
angle in the edge itself and a reduction of the ordinary teeth; such specimens are 
easily recognized by the irregularity in the lines of growth. 
10. Polygyra bicruris. (Tab. VII. figg. 8, 8 ac.) 
Helix bicruris, Pfr. P.Z.8. 1857, p. 109*; Monogr. Helic. Vivent. iv. p. 815’. 
? Helix (Polygyra} bicruris (Pfr.), Fisch. & Crosse, Miss. Scient. Mex., Mollusca, i. p. 280, t. 12. 
figg. 13, 13 a-c’. 
Hab. N.W. Mexico: ‘Tres Marias Islands (Forrer, Richardson); Ventanas, State of 
Durango (//6ge) ; Mexico, without nearer indication of locality (coll. Cuming 12). 
? Cutapas (Sallé ?). 
The specimens from Durango are more depressed than the figure given by Fischer and 
Crosse; and the upper external tooth is a direct prolongation of the other, as described 
by Pfeiffer, whereas in Fischer and Crosse’s figure the upper tooth is separated and 
looks as if it would come forth behind the other. ‘Therefore 1 am not sure if our 
species is really the same as that of Fischer and Crosse, more especially as the localities 
are widely separated. 
11. Polygyra richardsoni, sp.n. (Tab. VII. fige. 9, 9 ac.) 
Testa obstructe perforata, subdepressa, leviter striatula, fulvo-fusea, nitidula; spira breviter conoidea ; anfr. 5, 
convexiusculi, ultimus rotundatus, basi valde convexus, ad aperturam distinctius striatus, deflexus et 
constrictus ; apertura perobliqua, lunata, peristomate reflexo, albo; paries aperturalis plica inequaliter 
bicruri munitus, margo externus dente mediocri obtuso, superne in laminam margini parallelam elongato, 
margo basalis dente uno mediocri, subcompresso, verticali. 
Diam. maj. 9-11, min. 8-91, alt. 4-5 ; apert. diam. 54, lat. obliq. 5} millim. 
Hab. N.W. MExico: Presidio de Mazatlan, State of Sinaloa (fichardson). 
Near P. triodontoides, Bland, but with the umbilicus stil] more obstructed, only the 
last part of the penultimate whorl visible within it (as in P. ariadne), the basal and 
external tooth nearer to one another, and the latter more distinctly prolonged upwards 
into a lamina. 
