100 MAIilXK AXD FISHERIES 



5 GEORGE v., A. 1915 



shell are unmistakable characters. The long spire and delicate shells are in conform- 

 ity with its protected habitat. It varies in length of spire, conxevity of whorls 

 and size and shape of the aperture and the axis is often considerably twisted. Its 

 food consists of algir. (PI. X, Fig. 4). 



Genus Planorbis. 



Seven species were identified. Of these, three, P. bicarinatus, P. trivolvis and 

 P. car7}pa7iulatus belonging to a large-shelled group inliabiting muddy bays, possess 

 comparatively high, sinistral shells and certain common characters in respect of 

 the reproductive organs which will be dealt with in a subsequent paper. The 

 others belonging to a small-shelled group with a more varied habitat possess low, 

 flattened, dextral shells and, as far as examined, certain other characters in the 

 reproductive -organs. This group includes P. hirsutus, defiectus, exacuous and 

 dilatus, the range of distribution of which varies with each species. 



Planorbis bicarinatus, Say, occurs abundantly in weedy sand runs and weedy 

 muddy bays; found also on rocks near the latter. Distinct and easily recognized 

 by the two prominent angular carinae. Aperture slightly oblique, somewhat 

 triangular, broadly rounded below, lip thin with varical thickening behind. Shell 

 bi-concave, lower concavity with the sides interrupted by the carinae, upper smooth 

 and funnel-like. Former apertures often evident on the body whorl as pronounced 

 transverse ridges with darker periostracum. The possession of a broad high shell, 

 which is carried on edge and has an aperture only slightly oblique, doubtless in- 

 dicates for this form sheltered rather than exposed situations. In the young the 

 carinae as pronounced but the aperture less oblique than in the adult. Food con- 

 sists of green algse. (PI. X, Fig. 5). 



Planorbis trivolvis, Say. ; found plentifully in protected muddy bays and some- 

 times along the sheltered shores of the inner islands. Prefers shallow bays with 

 comparatively high temperature. Found onlj^ in shore collections and water less 

 than 2 feet in depth. The largest Planorbis of the region (Adult measurements, — 

 width 20-30 mm, length 10-13 mm). Shell with shallow concavity above the 

 smooth slopes of which are interrupted by the carina of about the last half of the 

 body whorl. A deep umbilicus into which the rounded whorls disappear below. 

 Aperture large, triangular or rhomboidal in outline, narrower above, lip thin, 

 much flared Mith a varical thickening behind. One or more former apertures 

 evident. Lines of growth coarse. Not fitted for exposed situations because of 

 the size and shape of the shell and the vertical position in which it is carried. Varies 

 with age. Young lighter in color, shells high and narrow, while adults are much 

 broader than high. Recognized easily at all stages by the upper concavity of the 

 shell. Series showing all stages readily secured. Eggs laid in flat brownish capsules 

 on lily-leaves, sticks and even on other molluscs. Food, filamentous algse, diatoms 

 and desmids. (PI. X, Fig. 6). 



Planorbis campamdatus, Say. ; occurs abundantly in weedy places, both muddy 

 and sandy, up to the depth of at least three fathoms. Easily recognized by the 

 campanulate expansion of the body whorl a short distance behind the aperture and 



