MOLLUSCA. at 
The presence or absence of an operculum or lid, gives 
rise, in this:system, to‘a division of the univalves into two 
sections, and the families are established from circumstances 
connected with the number of the tentacula, and the num- 
ber and position of the eyes. The families amongst the 
bivalves are arranged according to the structure of their 
cloak or external covering. In the class of multivalves, 
which we have omitted in the table, the characters are taken 
from the form and structure of the shell. 
The work of Geoffroy, entitled Trazté Sommaire des Co- 
quilles tant fluviatiles que terrestres, qui se trouvent aux 
environs de Paris, 1767, is constructed upon the principles 
of Adanson. Here, however, the objects were not suffi- 
ciently numerous to admit of all the subdivisions of that 
author, but he has made the form of the animal subservient 
to the construction of generic characters. 
After these attempts to classify the animals which inhabit 
shells had been made in France, the celebrated zoologist of 
Denmark, O. F. Miiller, turned his attention to the same 
subject. In the Zoologia Danica, which contains his di- 
gested views of the subject, he employs, in the construction 
of his genera of univalves, the characters first used by Adan- 
son; but among the bivalves, besides the form of the tubes 
or syphon, he notices the construction of the branchize and 
the presence or absence ‘of a foot. 
To our knowledge of the animals which inhabit bivalves, 
Poli, in his expensive work, the History of the Shells of the 
Two Sicilies, made very important additions. In the con- 
struction of his families, which are six in number, he em- 
ploys merely the characters furnished by the syphon and 
foot. In the first family, the animal has two syphons and 
a foot; in the second, there is only one syphon and foot; 
