THOMPSON: SNAIL GENUS MARSTONIA


becomes ventrad in position and is usually constricted at the aperture, where it descends rather
noticeably (Fig. 17B, lateral profile). The aperture is oval in outline, broadly rounded at the pos-
terior comer when mature, and about 0.80-0.93 times as wide as high. The aperture is adnate to
the preceding whorl for a short distance along the parietal margin. The peristome is nearly
straight in lateral profile and lies at about 350 to the shell axis.
 Measurements in mm for specimens with a complete peristome are as follows holotypee
measurements in parentheses): shell length 2.0-2.5 (2.3), shell width 1.6-1.9 (1.75), aperture
height 1.0-1.1 (1.0), aperture width (0.8-1.0 (0.9).
 Occasional gerontic specimens reach a length of 3.3 mm and are proportionately more
slender than smaller specimens because of the ventrad position of the body whorl. Occasional
small males of about 1.8 mm long may develop a complete peristome.
 OPERCULUM (FIG. 25G).-The operculum is paucispiral, ovate, thin, transparent, membranous,
and light yellowish-white in color. It consists of about two whorls and is sculptured on the outer
surface with fine irregular incremental striations.
 FEMALE (FIGs. 18A-B).-The female reproductive system is similar to that of 11. lustrica and
related forms, except as follows: (1) the oviduct loop is vertical to the axis of the pallial oviduct;
and (2) the bursa copulatrix does not overlap the posterior end of the pallial oviduct and is com-
pletely imbedded in the posterior pallial oviduct, except for its distal tip.
 MALE (FIGS. 19A-C).-The vas deferens I is only moderately convoluted and is only slightly
thickened in the region of the convolutions. The prostate is relatively elongate and rather dis-
tinctly lobed, in contrast to the more concentrated prostate in XM. lustrica. The verge originates
on the nape on the right of the middorsal line and is forked distally. The penis is considerably
longer and more enlarged than the apical lobe. The latter is papilliform and bears a single, small
apical gland on the ventral side of its tip. A second small, raised gland is present on the ventral
surface of the verge about halfway between the base and the apical lobe.
 DISTRIBUTION (FIG. 20).--M. halcyon is confined to the lower half of the Ogeechee River
 System in eastern Georgia, where it is found in quiet water over a silt or sand bottom. It has been
 collected only in the Ogeechee River and Buckhead Creek, though it probably occurs in other
 creeks along the middle and lower course of the Ogeechee River.
 TYPE LOCALITY.-Buckhead Creek, 0.6 mi. W Miller, Jenkins Co., Georgia. HOLOTYPE:
 UF 22312; collected 6 October 1974 by Fred G. Thompson and Beverly E. Johnson. PARA-
 TYPES: UF 22313; same data as the holotype.
 At the type locality, Buckhead Creek is about 3-5 ft deep, with a hard sand bottom and a
 slight current. Snails were collected in quiet, shallow water on silty sand and in clusters of fila-
 mentous algae.
 SPECIMENS EXAMINED.-GEORGIA. BRYAN Co.: Ogeechee River, 1.1 mi. SE Blichton (UF
 22318); EFFINGHAM CO.: 1.7 mi. SW Guyton (UF 22316, 22319); JENKINS Co.: Buckhead Creek,
 0.6 mi. W Miller (UF 22311); SCREVEN Co.: Ogeechee River, 0.7 mi. SW Ogeechee (UF 22310,
 22314); Ogeechee River, 1.6 mi. SW Halcyon Dale; Ogeechee River, 3.0 mi. SW Oliver (UF
 22320).
 REMARKs.-This snail was always collected in open water, primarily on
 silty sand. Only rarely was it found on filamentous algae. On no occasion was
 it found on dead submerged leaves, moss, or other detritus, although other
 hydrobiids (Amnicola sp. and Lyogyrus sp.) were abundant on such substrates.
 Goodrich (1939:130) recorded four species of hydrobiids from the Ogee-
 chee River. His material was not available to me for examination, as it is being
 studied by another investigator. I cannot state the name(s) under which Good-
 rich recorded M. halcyon.
 ETYMOLOGY.-In the area where M. halcyon was found, the Ogeechee
 River is gentle and quiet, bordered with large cypress trees and hardwood
 forests. The species name is borrowed from the Greek halcyon, a mythological
 sea bird that brought peace and serenity wherever it alighted. The name is
 suggested by one of the places near where it was collected, Halcyon Dale.


1977