COKCHOLOGY. 



figure in all shells, being either round, 

 semi-ovate, lunate, or elongated, in diffe- 

 rent kinds. Byssiis, the appendage called 

 the beard ; by means of which some bi- 

 valves fasten themselves to the rocks. 



Mitlti-vatves. The shells of this order 

 are few, compared with either of the pre- 

 ceding; and the terms proposed for those 

 are applicable for the most part to the 

 rimlti\ alves. The following require more 

 explicit mention. Base, that part of the 

 shell upon which it rests : in the lepas 

 tribe, it implies the part immediately 

 seated upon the stem or pedicle ,- in the 

 balani, the base is generally larger than 

 the summit, and is the bottom, by means 

 of which the shell is fixed upon the 

 rocks or other extraneous bodies. Liga- 

 ment, the substance, whether membra- 

 naceous or tendinous, which serves to 

 connect the valves together. The con- 

 nexion of the valves in some multivalves 

 is formed by the parts of one valve lock- 

 ing into another. Operculum. The ba- 

 lani have the aperture at the summit 

 closed by means of four small pieces or 

 valves, which are commonly called the 

 operculum ; these opercula of the balani 

 are, however, very different from those 

 of univalve shells. 



We here introduce descriptions, illus- 

 trated by figures, of a few of the land and 

 fresh water shells of the United States, 

 induced particularly by the silence with 

 which these productions of our country 

 arc regarded in the Systema Naturz. In 

 the extensive work of Lister, entitled 

 " Historic sive Synopsis methodicx Con- 

 chylionim," &c. several of our shells are 

 figured, and, to a few of them, short de- 

 scr.ptions are annexed, not however de- 

 signated by specific names ; of this work 

 we have availed ourselves, from quotations 

 and references. As we have added 

 figures of all the shells here noticed, it 

 was not thought necessary to enter very 

 much at length into the description of 

 them. 



It will be readily perceived by the 

 Conchologist. that in the arrangement we 

 have deviated from the course pursued 

 in this work relative to the inviolability 

 of the Linnxan system, so as to introduce 

 some of the more recent improvements 

 in the construction of genera, and that 

 some considerable modifications are ven- 

 tured to be made in this article. 



UNIVALVES. 

 GENUS HELIX. 



Shell subglobose, suborbicular, broad- 

 er than long, spire convex ; aperture 

 VOL. IV. 



wider than long, diminished above by the 

 convexity of the penultimate whorl. 



Observation. The far greater number 

 of the shells belonging to this genus are 

 terrestrial, often inhabiting moist places ; 

 they are thin, brittle, and translucent ; 

 the young shells umbilicate almost in- 

 variably. 



1. H. JUbolabri*. Shell thin, fragile 

 conve^, imperforated ; with six volutions, 

 whorls obtusely wrinkled across, and 

 spirally striated with very fine impressed 

 lines, a little waved by passing over the 

 wrinkles, both becoming extinct to- 

 wards the apex, which is perfectly 

 smooth ; aperture lunated, not angulated 

 at the base of the column, but obtusely 

 curved, lip contracting the mouth ab- 

 ruptly, widely reflected, flat and white. 



Length of the column, three-fifths of 

 an inch ; breadth one inch. 



Plate 1. fig. 1. 



Lister conch, tab. 47 ? 



Rhodia ? Gmelin's Edit. Syst. Nat. 



The common garden snail, frequenting 

 moist shaded situations, and is generally 

 well known. It is very probable this is 

 the Rhodia of Authors, but as in the des- 

 cription of that species nothing is men- 

 tioned of the reflected lip, and not hav- 

 ing in our possession the vol. of Chemn. 

 conch, referred to for a figure of it, we 

 have made an interrogative reference, 

 and for the present have adopted a new 

 name. An umbilicate shell that very 

 much resembles this species is some- 

 times found, but in consequence of not 

 having a perfect specimen, we have not 

 figured it ; it is much less than Jllbolabriti 

 breadth about four-fifths of an inch, the 

 lip is reflected, but not flat, and on the 

 pillar lip is a strong tooth placed oblique- 

 ly, as in H. Tridentata : we have called it 

 Helix Thyrridiu. 



2. H. Arboreta. Shell very thin, fra- 

 gile, depressed, horn colour, pellucid, 

 very little convex ; whorls four, irregu- 

 larly wrinkled across ; aperture sublunat- 

 ed, lip thin, brittle, junction with the 

 body whorl acute ; umbilicus large and 

 deep. 



Length one-tenth of an inch nearly; 

 breadth nearly one-fifth. 



Plate 4. fig. 4. 



Under the bark of decaying trees very 

 common. Inhabitant pellucid ; base white, 

 acute behind, not extended forward b 



B 



