266 APPENDIX. 



Length about one and nine-tenths, breadth two and nine- 

 tenths of an inch. 



This shell exhibits a remarkable appearance, in the un- 

 usually great convexity of the disks and umbones. It is 

 strikingly distinct, and was presented to me by Captain 

 Le Conte of the Topographical Engineers, who informs mc 

 that it is an inhabitant of South Carolina. PI. 14, fig. 3, 4. 



D. CLASS VERMES. 

 ORDER CRYPTOBRANCHIA. 



HIRUDO. 



1 . H. parasitica. A yellow vitta before ; quadrate mar- 

 ginal spots each side ; beneath with about eleven longitU' 

 dinal lines ; ocular points two. 



Descr. Body dilated when at rest, narrowed before; 

 above varied with dull-yellowish and blackish-brown ; a 

 yellow vitta commences at the anterior extremity and is 

 more or less elongated, in some specimens less than one- 

 fourth the length of the body, and in others extending 

 nearly or quite to the posterior disk ; lateral margin with 

 eighteen or twenty symmetrical equal and equidistant 

 quadrate yellowish spots; posterior disk above radiate 

 with yellowish ; ocular points two, approximate, sometimes 

 apparently confluent ; beneath very flat, whitish, with about 

 eleven longitudinal lines ; lateral edges very acute. 



Length in a state of repose two inches, greatest breadth 

 seven-tenths of an inch. 



This leech is frequent in the lakes of the north-western 

 region, adhering to the sternum or inferior shell of Tor- 



