210 AMERICAN JOURNAL 



length, then run diagonally to the outer edge of the mem- 

 brane. 



The narrowing of the membrane towards its extremity renders 

 any enumeration of the number of teeth in any transverse row 

 more than usually unsatisfactory. 



The central teeth are on long, narrow plates, squarely trun- 

 cated above and also below, 



where they are somewhat Fig. 3. 



widened. These plates are 

 delicately fringed or crimp- 

 ed, both at the upper and 

 lower edge of attachment, 

 as are also the plates of the 

 laterals and uncini ; but, as 

 this fringing or crimping is 

 very delicate, we have not 

 attempted to show it in the 

 figures. The plates of the 

 central teeth bear upon their Achaiina virginea. 



whole length a stout, broad 



denticle, wide as the plate itself at its upper edge, thence very 

 gradually narrowing towards its base, from beyond which it pro- 

 jects into a long, blunt, stout, lance shaped point. 



The laterals are but two in number on each side of the centrals. 

 They are of the same shape as the centrals, but are slightly 

 wider, and the lance-like projections are shorter. They are al- 

 most equally symmetrical as the centrals, still the points of the 

 projections are decidedly thrown towards the centrals, and the 

 lower termination of the plates are more widely expanded on the 

 side farther from the centrals. By these slight differences the 

 laterals are made to bear the usual unsymmetrical appearance* 

 in relation to the centrals. 



There are two distinct forms of teeth among the uncini. The 

 first form is borne by only the three teeth next the laterals. The 

 plates of these are much broader than those of the laterals. The 

 lance-like point of the laterals is replaced by two irregularly- 

 shaped denticles, the outer one larger than the other. These are 

 very variable in shape, and are smaller on the outer of the three 

 uncini which bear them. In their most highly developed state 

 they bear some rude resemblance to a mitten. 



* In most of the Geophila the laterals on either side of tlie median line 

 differ from the centrals by the suppression of a part of the centrals oa 

 their respective inner sides. Where the centrals are tricuspid, the late- 

 rals are either bicuspid or the third cusp of the centrals is suppressed 

 wholly or in part on the side nearer the centrals. 



