CIRCULAR. 



It is intended that this work shall embrace all our species hitherto described 

 by American and European authors; and as the geographical distribution is 

 a matter of great importance, and one which has not been sufficiently noted, 

 the author will be greatly indebted to those who take an interest in this sub- 

 ject, if they will forward him catalogues of the species, or specimens of the 

 univalve shells found in the streams with which they may be acquainted. 



Descriptions of the animals and their habits would also be very useful; and 

 for any assistance given, acknowledgements will be made. 



The soft parts or "animal" of lo SPIXOSA, LEA, or a description, would be a 

 desideratum. 



Communications may be addressed to the author, at Marietta or Columbia. 

 Pennsylvania, or to the care of Mr. DOBSON, Philadelphia. 



CHARACTERS OF NEW MOLLUSCS AND PARASITIC ANIMALS. 



Anculosa littorina. 



A. testa solida, conica, olivacea; lineis transversis cincta: anfractibus qua- 

 ternis planulatis: apice erosa: sutura vix excavata: apertura subrotundata, 

 superne angulata. 



HAB. Holston river, Virginia. Length, ^ inch. 



Cerithium (Potamis) Californicum. 



Testa solida, turrita, rufescente: anfractibus novenis convexis; costulis lon- 

 gitudinalibus; lineis transversis costulas decussantibus: sutura impressa: aper- 

 tura rhqmboidea, purpurea; labro crasso. 



HAB. California, in brackish water. Mr. Nuttall. Length, 1 inch. 



Cyclas elevata. 



Shell orbicular, cardinal tooth prominent, lamellar tdeth thick: beaks ele- 

 vated. Color brownish-olive. Length 0.55, height 0.50 in. 

 HAB. Near New Orleans. 



Hirudo (Clepsina) scabra. 



Head distinct, pointed, provided with a sucker and two eyes: body with 

 about four longitudinal rows of equidistant points or spines; a large posterior 

 sucker. Color light brown. Length | in. 



Found upon Planorbis bicarinatus. 



Cercaria hyalocauda. 



Body dark brown or blackish, about as long as the tail: tail transparent, 

 tapering, and suddenly diminished at its junction with the body. Just visible 

 to the naked eye. Parasitic upon Physa heterostropha. 



Genus Discus. 



I propose to establish a genus under this title, for the reception of Planorbis 

 armigerus, Say. Its characters are the same as in Planorbis; with the addi- 

 tion of the teeth, situated within the aperture of the shell. 



