CONCHOLOGY. 



figure in all shells, being either round, 

 semi-ovate, lunate, or elongataed, in dif- 

 ferent kinds. Byssus, the appendage 

 called beard ; by means of which some 

 bivalves fasten themselves to the rocks. 



Multi-valves. 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 

 multivalves. 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. Ligament, 

 the substance, whether membranaceous 

 or tendinous, which serves to connect 

 the valves together. The connexion of 

 the valves in some multivalves is form- 

 ed by the parts of one valve locking 

 into another. Operculum. The balani 

 have the aperture at the summit closed 

 by means of four small pieces or valves, 

 which are commonly called the opfercu- 

 lum ; these opercula of the balani are, 

 however, very different from those of uni- 

 valve 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 

 are regarded in the Systema Naturae. In 

 the extensive work of Lister, entitled 

 " Historic sive Synopsis Methodicae Con- 

 chyliorum," &c. several of our shells are 

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

 scriptions are annexed, not however de- 

 signated with specific names ; of this work 

 we have availed ourselves, from quota- 

 tions and references. 



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 Linnaean 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. 



We think it proper to state, in addi- 

 tion to the above remarks, which were 

 annexed to the descriptions in the first 

 and second American editions of this 

 work, that several species and three new 

 genera are now added, which, with the 

 exception of a small number of new spe- 

 cies now first published, we have previ- 

 ously given to the world, in several de- 

 tached essays, in the pa^es of the Joicrtuil 



VOL. IV. 



of the Academy of Natural Sciences, in the 

 collection of which Academy all the spe- 

 cies are preserved. 



UNIVALVES. 



GENUS HELIX. 



Shell subglobose, suborbicular, broad- 

 er than long, spire convex ; aperture 

 wider than long, diminished above by the 

 convexity of the penultimate whorl. 



Observation. The shells belonging to this 

 genus are terrestrial,often inhabiting moist 

 places ; they are thin, brittle, and trans- 

 lucent ; the young shells are umbilicate al- 

 most invariably. 



I. H. Mbolabris. Shell thin, fragile ; 

 convex, imperforated ; with six volutions, 

 whorls obtusely wrinkled across, and spi- 

 rally 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 abrupt- 

 ly, 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 



2. H. Jlrboreus. 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 5 

 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 be- 

 fore the head ; head and neck dusky ; 

 tentaeula four; lower ones very short; 



B 



