566 HISTORY OF SYSTEMS. 



with 2 lips on each side : rectum traversing the heart : ad- 

 ductor muscles, one or more, for closing the shell : shell 

 2-valved ; valves attached to each other by a hinge on one 

 side, the other side and the two ends free. Marine, rarely 

 fresh water ; breathing water. 



Order I. CLADOPODA. Animal adductor muscle poste- 

 rior : mantle, sides soldered, front open : foot 

 thick, clubbed, passing out in front : shell gaping 

 in front, opened by an external adductor muscle, 

 generally protected by accessory pieces, and closed 

 by one adductor muscle : elastic ligament none. 

 Gen. Pholas, Teredo, Aspergillum. 



Order II. PACHYPEDA. Animal adductor muscles 2, 

 distinct, anterior and posterior : mantle side sol- 

 dered, front open : foot thick, passing out in front : 

 shell regular, mostly inequivalved, gaping at the 

 ends : elastic ligament internal, in a tooth. Mya, 

 Corbula. 



Order III. LEPTOPODA. Animal adductor muscles 2, 

 distinct, anterior and posterior : mantle side open : 

 foot small, compressed : shell regular, generally 

 equivalved, gaping, more or less, at the ends : 

 elastic ligament internal, in a tooth. Mactra, 

 Nucula. 



Order IV. PHYLLOPODA. Animal adductor muscles 2, 

 distinct, anterior and posterior : mantle side open : 

 foot small, lamellar : shell regular, equivalve : elas- 

 tic ligament marginal, linear, external. 

 (a.) Solen, Psammobia, Tellina. 

 (b.) Cyclas, Venus, Cardium, Tridacna, Chama, Pectun- 



culus, Trigonia, Unio. 



Order V. POGONOPODA. Animal adductor muscles 2, 

 anterior and posterior : mantle side free : foot very 

 small, with a bundle of byssus in front : byssus 

 ending in suckers : shell equivalve : elastic liga- 

 ment marginal, linear. Area, Mytilus, Avicula. 

 Order VI. MICROPODA. Animal adductor muscle one (or 

 two or three close together), nearly central : mantle 

 side free : foot very small : shell mostly inequivalve : 

 elastic ligament internal, short, in a hollow ; not 

 marginal. Pecten, Ostrea, Anomia. 



Remarks. " So little is known of the animals of this 

 class individually, that it may, perhaps, be better to arrange 

 them by the shell, elastic ligament, and muscular impressions 

 only ; as in the following manner : 



" a. Elastic ligament none : adductor muscle external : 



