246 PROCEEDINGS OF THE ACADEMY = OF 
points, and a stout middle cusp bearing a stout cutting point which 
reaches to the lower edge of the base of attachment. The lateral 
teeth are longer and broader than the central tooth; they are un- 
symmetrical by the suppression of the inner cusp and cutting point, 
and the lower half of the base of attachment being thrown off 
towards the outer edge of the membrane, though its inner lower | 
lateral expansion is not suppressed; the outer side cusp is well 
developed and bears a short distinct cutting point; the inner 
cusp is very stout and bears a very stout cutting point extending 
slightly beyond the lower edge of the base of attachment. The 
change from the lateral to the marginal teeth is formed by the 
lesser size of the reflected portion and the greater development of 
the inner cutting point, as well as by the blunt bifurcation of the 
outer cutting point. The marginal teeth have their base of attach- 
ment long and narrow, quadrangular, curving outward, prolonged 
above the reflection, which is small, but bears a highly developed 
cutting point obliquely and bluntly bicuspid on its outer edge, 
the inner division much the larger. The number of perfect 
laterals varies somewhat. I counted seven in c2trina, eleven in 
planilabrum, ten in abbreviata and amanda, eight in umbilicata, 
virginea, and bilineata, five only in gracilis. The number of 
marginal teeth also varies, but they are numerous in all the species; 
in virginea I counted over one hundred and twenty. Excepting 
that some of the membranes had narrower teeth than others, I 
found no difference in them. They all agree (including those of 
the species named on p. 244) with the figure given by Heynemann 
(Mal. Blatt. 1867, t.i. fig. 1-la.) of the dentition of P. lirata. 
The genital system of one of the undetermined specimens is 
given on pl. XIX., fig. 1. The ovary (ov.) is small and stout; the 
epididymis (ep.) is short and greatly convoluted at the end nearer — 
the oviduct; the testicle (¢.), composed of short cceca, is small 
and embedded in the upper lobe of the liver in the very apex of 
the shell; the oviduct (ovzd.) is long, convoluted ; the vas deferens 
(v. d.) enters the prostate high up on the oviduct, not at its lower 
end, as usual; it runs down to the external orifice, then up to near 
the end of the penis sac, where it enters ; the vagina (v.) is long, 
greatly swollen at the entrance of the duct of the genital bladder ; 
the last mentioned organ (g. b.) is small, with a short duct which 
enlarges greatly before entering the vagina; the penis sac (p. s.) 
is large, long, bluntly terminating, with a decided constriction 
