74 CONCHIFERA. MONOMYAIRA. 



tachment ; shell marked interiorly with two separate and lateral 

 muscular impressions. 



It is not necessary to detail here all the arrangements pro- 

 posed for this class of animals. They were included by Lin- 

 nseus among his Vermes Testacea, and form the class of Mol- 

 lusca Acephdla in the Regne Animal of M. Cuvier. The older 

 naturalists, who arranged the testaceous animals as one great 

 family by the form of their testaceous covering, took their cha- 

 racters wholly from the shell ; and this department of science, 

 including the testaceous coverings of the preceding class, formed 

 the branch of science termed Conchology. 



ORDER I. MONOMYAIRA. 



Animal with one muscle of attachment ; and the shell with one 

 subcentral muscular impression. 



This order is divided by Lamarck into seven families as follows : 

 SECTION I. Ligament either none, or unknown, or replaced by a tendinous chord. 



a. Shell adhering either immediately or by a tendinous chord, which serves in place 

 of a ligament ; animal with two opposite, ciliated and cirrhous arms. Brachiopoda. 



b. Ligament and animal unknown ; shell inequivalve. Rudista. 



SECTION II. Ligament not marginal, inclosed in a short space under the beak, al- 

 ways perceptible, and not forming a tendinous tube under the shell. 



a. Ligament interior or partly so ; shell irregular, foliaceous, sometimes papyrace- 

 ous. Ostracea. 



b. Ligament interior or partly so ; shell regular, compact, not foliaceous. Pectinides. 

 SECTION III Ligament marginal, elongated on the margin, sublinear. 



a. Shell longitudinal or subtransverse, with the muscular impression in an isolated 

 space, not marginal. 



* Ligament at the inferior margin of the shell, or divided. Malleacea. 



** Ligament at the lateral border of the shell, and always entire. Mytilacea. 



b. Shell transverse, equi valve, with the muscular impression elongated, bordering 

 the superior limb. Tridacnites. 



SECTION I. 



Ligament none, or unknown, or replaced by a tendinous chord 

 which supports the shell. 



FAMILY I. BRACHIOPODA. 



Two opposite, elongated, ciliated arms near the mouth, rolled 

 up in a spiral form in repose ; mantle with two separate lobes 

 before, enveloping the body. Shell bivalve, adhering to ma- 

 rine bodies, either directly or by means of a tendinous chord. 



The shells of this family are more or less inequivalve, and open by a hinge. The 

 true ligament of the valves is not known ; and the tendinous chord which attaches the 

 shell appears to be a prolongation of the muscle of attachment. The shell some- 

 times adheres to marine bodies by its lower valve, and sometimes by a tendinous chord. 



