E. MAESHALLT. — GEN. CHARACTERS. 91 



at some point on tlie ledge itself and leading internally into the 

 gastral cavity. 



The fine dermal laiticeivork is scarcely discernible by the 

 naked eye but under a hand-lens may be seen to extend all 

 over the external surface, except on the thin iris-like oscular 

 membrane. The nodes of the delicate lattice generally appear as 

 minute whitish spots. Through the layer are everywhere to be 

 seen, in varying degrees of distinctness, the entrances into the 

 incurrent canals. These are of all sizes under 1 mm. diameter. 



A cuff is usually more or less distinctly present. In several 

 specimens, it was plainly developed for only a portion of the 

 sieve-plate circumference and merely suggested for the rest. It 

 is as a rule directed obliquely upwards ; exceptionally, however, it 

 becomes nearly horizontal. Its free edge is always uneven and 

 and occasionally even deeply indented, thus giving unequal 

 breadth to the cuff in different parts. Measured on the upper 

 surface, the breadth may be as much as 10 mm., but generally 

 is much less. On the lower side, the cuff is joined by the parietal 

 ledges which run between the last oscula at this end and are 

 here comparatively low. Besides being somewhat thinner, the 

 cuff differs from the average crests of the parietal ledge in being 

 rather sharp-edged instead of being rounded, and in showing a 

 fringe of fine, inconspicuous marginalia, projecting to a length 

 of less than 1 mm. But this difference is of no essential im- 

 portance, since certain parietal crests have been occasionally 

 found having characteristics exactly like those of the cuff. 



The sieve-plate may show in some cases a convexity nearly 



