MOLLUSCA, 37 



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 OeofFroy, entitled Traite Sommaire des Co- 

 quilles tant flumatiles que terrestres, qui se trouveni 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 branchiae 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, tfiere is only one syphon and foot; 



