238 PROCEEDINGS OF THE ACADEMY OF [1893. 



stronger and somewhat irregularly distributed, irregularly denticu- 

 lating the superior and inferior or cutting edges. In fig. I, Plate 

 VIII, they are exactly drawn from a jaw of V- pulchella, enlarged 

 about 300 diam., and it is distinctly seen that one rib stands 

 regularly on either side of the not otherwise marked median line, 

 being more distant in the superior part, then approximating to 

 near the cutting edge. The other ribs, about 12 on either side, 

 stand somewhat irregularly, sometimes in pairs. Near the ends 

 of the jaw they become, rather abruptly, quite fine, visible only 

 under strong enlargement, so that otherwise these parts appear to be 

 smooth. Whether this formation of the jaw is derived from a 

 goniognathous one in the embryonal stage remains to be determined, 

 but it is probable, the more so if we compare it with the jaw of V. 

 parvida (Plate VIII, fig. K). There the ribs, especially those on 

 the middle of the lateral parts, were not simply and symmetrically 

 raised, but their sides are steep, abrupt outward, more gently slop- 

 ing inward. Thus they give the impression of plates originally sep- 

 arated and grown together by subsequent growth. 



In the middle and upper part of the jaw, very fine, dense, some- 

 what irregular, horizontal "stride," apparently undulating, evidently 

 lines of growth, corresponding with the ribs, are visible. 



From the main plate or jaw proper, a rather strong posterior 

 plate springs from the whole inferior or cutting edge, extends back- 

 ward without interruption and tapers into the long and strong 

 hyaline tenaculum membrane of the jaw. It evidently serves to 

 reinforce the jaw, and especially the cutting edge. Plate VIII, fig. 

 I, shows it as seen from the front through the main plate, from a to 

 b, on one side, both its contours being plainly visible. When the 

 jaw is resting on its front side the posterior plate may conceal the 

 cutting edge from sight, and then the latter appears to be quite 

 even, not denticulated, especially so in the median part. How far 

 this formation of the jaw is present in other land-pulmonates, I do 

 not know — very probably there are many intermediate stages. In 

 Patula striatella I have found it almost exactly the same as in Vallonia 

 and in the Zonitidce ; in Zonitoides ligera Say, there is a distinct 

 indication of it, though with some modifications. 



The jaw, especially in the species where it is comparatively stout, 

 presents quite different aspects, according to the side and direction 

 from which it is viewed. Thus we may understand partly the fig- 



