SPILERIUM OF NORTH AND SOUTH AMERICA. 



The genus Sphserium was characterized under its present name by Scopoli, 

 in 1777 ; since that time, however, it has received various denominations, 

 and the one under which it has been most generally known, that of Cyclas, 

 was applied to it in 1792 by Bruguiere. 'Mr. Gray revived the term of Sphse- 

 rium in 1847, and his example has been followed by the conchologists of the 

 continent of Europe. I was the first in this country to discard the name of 

 Cyclas for that of Sphserium. 



The species composing this genus are small bivalves inhabiting rivers, 

 lakes, streams, and still waters ; they are plentifully distributed all over the 

 globe, but as far as present experience goes, seem to be more abundant on 

 the northern portion of this hemisphere than elsewhere. 



The shell is transversely oval, nearly equilateral, thin, fragile, sometimes 

 translucent, with beaks more or less raised ; its entire surface is transversely 

 striated and covered with a light epidermis varying in color ; the margins are 

 rounded, obtuse or angular. The interior of the valves is smooth and varies 

 in color ; the muscular impressions are not very distinct ; the posterior one 

 is slightly the largest ; the palleal impression is parallel with the basal mar- 

 gin ; it is narrow and always simple. The hinge margin is very variable ; it 

 is usually composed of two small teeth in each valve ; at times, however, they 

 are single in one and double in the other, or else single in both valves ; these 

 teeth are occasionally rudimentary, or even nearly obsolete. The lateral 

 teeth placed on each side of the cardinal teeth are double in the right valve 

 and single in the left one ; the anterior lateral tooth is usually the shorter. 

 The ligament is external ; it is short, not very conspicuous, and is always 

 found on the longer portion of the shell. 



The animal of Sphserium. has a, broad foot, capable of considerable exten- 

 sion ; it uses it either to bore holes in the mud, in which it sinks the poste- 

 rior portion of the shell, or as means of locomotion. The syphonal tube is 

 double and very retractile ; it is often white like the foot but at times it is 

 colored. 



The habits of these molluscs are very similar to those of Pisidium, with 

 which they are often found living. The species of Sphserium are less abun- 

 dant in individuals than those of Pisidium ; they are also less generally dis- 

 tributed, and are more confined to certain localities than the latter. 



Fam. CYCLADES Fer. 

 SPHSERIUM Scopoli. 



Fectunculus, Lister, 1685. Musculus, Gault. 1742. Tellina, Linn., 1758. 

 Sphcerium, Scop., 1777. Cardium, Da Costa, 1778. Cyclas, Brug., 1782. Nux, 

 Humphr., 1797. Musculium, Link. 1807. Cornea, Pisum, Megerle. 1811. 

 Corneocyclas, Fer., 1818. Amesoda, Rafin., 1820. Pisidium, Verany, 1846. 

 Cycladites, Krug, 1848. 



From Proceeding of the Academy of Natural Sciences of Philadelphia. Dec. 1861. 



