266                                       APPENDIX.
Length about one and nine-tenths, breadth two and nine-tenths of an inch.
This shell exhibits a remarkable appearance, in the unusually 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 me 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 marginal spots each side; beneath with about eleven longitudinal 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-