534 HISTORY OF SYSTEMS. 



Les Mactracees. Les Corbulees. 



(") Ligament solely external. 



Les Lithophages. Les Nymphacees. 



(6) Shell close at the sides when the valves are shut. 



*** Conchiferes lamellipedes. Foot flattened, lamellar, 



not posterior. 

 (2) Shell irregular, always inequivalve. 



Les Conques. Les Cardiacees. 



Les Arcacees. Les Nayades. 



Les Camacees. 



Order II. CONCHIFERES MONOMYAIRES. One adductor 

 muscle only ; and its impression on the valves is single 

 and subcentral. 



(1) Shell transverse and equivalve. 



Les Benitiers. 



(2) Shell either longitudinal or inequivalve. 



(a) Ligament marginal, prolonged along the mar- 

 gin, and sublinear. 



Les Mytilacees. Les Malleacees. 



(b) Ligament concentrated in a short space under 

 the beaks, always recognisable and not tubiform. 



Les Pectinides. Les Ostracees. 



(c) Ligament either unknown or forming a tendi- 

 nous tube under the shell. 



Les Rudistes. Les Brachiopodes. 



Class MOLLUSCA. (June, 1819.) 



Order I. PTEROPODES. No foot to creep upon, nor arms 

 to drag themselves along, or therewith to seize 

 their prey. Two opposite and similar fins adapted 

 for swimming. 



Order II. GASTEROPODES. The body straight, never spiral 

 nor enveloped in a shell that can contain it. A 

 muscular foot united to the body throughout its 

 length and forming a ventral disk to creep upon. 



Order III. TRACHELIPODES. The body twisted spirally, 

 detached from the foot, and always enclosed in a 

 spiral shell. The foot free, flattened, attached to 

 the inferior base of the neck, and fitted for creeping. 



Order IV. CEPHALOPODES. The body contained interiorly 

 in a sacciform mantle. Head protruding from this 

 sac, crowned by non-articulated arms, which are 

 furnished with suckers and encircle a mouth armed 

 with two horny mandibles. 



