426 Illinois State Laboratory of Natural History. 



terior to a vertical line passing through the muscle im- 

 pressions.) The posterior part is considerably broader 

 than the anterior. 



Seen from above, the shell is sharply and concavely 

 pointed anteriorly, then quickly broadens and remains 

 of the same width nearh^ to the posterior end, which is 

 convexly and bluntly pointed. There is a constriction 

 or sinus (PI. XXXIX., Fig. 13) just in front of the 

 middle, which, in the position indicated in Fig. 1, PI. 

 XL., is seen to be double. The deeper sinus is just 

 anterior to the middle and above the muscle impres- 

 sions. Anterior to this is a small conical elevation 

 separating it from another smaller sinus or constriction 

 from which the surface slopes quite quickly to the ante- 

 rior part. 



The radiating hairs of the anterior and posterior 

 parts are more abundant than in L. reticulata, and 

 resemble those of Z. neocomensis. The anterior hyaline 

 flange is three times as wide as the posterior one. 

 (PI.'XXXIX., Fig. 8.) 



Muscle impressions four, elongate-oval, situated at the 

 bottom of a circular depression just anterior to the mid- 

 dle of the shell and just above the deepest part of 

 the ventral sinus, their long axes parallel to that of the 

 shell. There are two or three similar depressions in the 

 same vicinit3^ 



Terminal segment of the first antennae (PI. XL., Fig. 

 6) four times (m female) or five times (in male) as 

 long as wide, armed apically witih three setae, the terminal 

 one of which is divided apically as a "sense seta" and is 

 once and two thirds the length of the segment ; the other 

 two equal, and one fourth longer than the segment. The 

 penultimate segment is a fourth longer than the last one, 

 while the antepenultimate is but half as long as the penul- 

 timate. The one preceding this, and also the basal seg- 

 ment, are ciliated along their inner edges (PI. XL., Fig. 6). 



The second antenna of the male differs from that of 



