468 TERMINOLOGY OF 



a sinus running, bay-like, into the middle of the valve from 

 this marginal border (Fig. 88 c). The spots marked a, a, 

 are called the muscular impressions, or scars ; the border, b, b, 

 is the pallial margin, or submarginal impression, and the sinus, 

 e, is the pallial sinus, or linguiform impression. 



The shells which have two muscular impressions, one to- 

 wards the anterior and another towards the posterior slope, 

 are called Dimyaires, or bimusculous ; and the shells which 

 have one central or subcentral impression, are called Mono- 

 my aires, or unimusculous.* The impressions are made in the 

 shell by the attachments of the transverse, or adductor muscles 

 or muscle, which pass direct from one valve to the other. 

 You will remember that the office of the hinge-cartilage is to 

 open the valves, and the office of the transverse muscles is to 

 close them and to prevent their being opened too wide asunder 

 — two duties in one office, for which they are purposely con- 

 structed, partly of muscular fibre and partly of ligament. 

 The animal, when at rest and submerged, keeps the valves 

 open to a certain extent, to allow a pleasurable expansion of 

 its soft organs ; but when alarmed, or deprived of water, it 

 shuts them close with a vigorous effort. The closure is 

 effected by the adductor muscles, which, being voluntary 

 muscles, get weary of their tension and relax their gripe. 

 Then their valves open just far enough to place these muscles 

 in a state of relaxation at their ease, but not so far as to ex- 

 pose too much the soft body they enclose, the gape proper 

 to each species being retained by the ligament adnate to the 

 adductor muscles counteracting the elasticity of the hinge 

 cartilage. The ligament and muscle are not intermingled, 

 but only in juxtaposition. " This ligament is placed on 

 the inner side, close to, or partly attached to, the adductor 

 muscle, as may be observed by cutting the body, usually so 

 called, across, when the two substances will be most dis- 

 tinctly visible, the one muscular and the other eminently fi- 

 brous and pearly." That the valves are kept from separating 

 beyond the just limits by the ligament is proved by the cir- 

 cumstance, that when the animal is dead, and the muscle is 

 in a nearly decayed state, the distance is nevertheless re- 

 tained, nor is made wider when the muscular part is cut 

 through ; but instantly on dividing both muscle and liga- 

 ment, the valves fly back suddenly as far as the mechanism of 



* Only a very few of the Monomyaires are, strictly speaking, unimuscu- 

 lous. Besides the large subcentral or posterior imjuession, there is in most 

 of them a small impression near to and below the hinge. In Avicula there 

 are several of these minute impressions. 



