239 



limestone, at Section 7; between the Trilobite beds and the 

 Strophalosia bed. in Avery's Creek, and on the Lake Shore 

 (not nnc'omnion). 



Note. — This species is. at Eighteen Mile Creek, almost 

 entirely confined to the Encrinal limestone; bnt at Morse 

 Creek, near Atliol Springs, it is common in the shales below 

 the limestone. 



Class Pelecypoda. Golufuss. 



{LamcUihranchiatd. Bhiiuville.) 



The Pelecj'poda or Lamellibranchiata are marine or freshwater mol- 

 luscs, with a bivalve shell. The valves are complementary, and are iu 

 the majority of species of nearly similar outline and size. In each valve 

 may be distinguished an initial point, or beak, around which the con- 

 centric li7ies of growth mark the successive additions of shelly matter. 



The orientation of most shells is eifected by holding them with the hinge 

 line uppermost and the beaks pointing away from the observer. Thus 

 placed, the upper border is the dorsal and the lower the ventral border. 

 The end farthest away from the observer is the anterior end ; that nearest, 

 the posterior end. The valves are designated as the right and left valves, 

 respectively. The articulation of the valves is commonly effected by the 

 interlocking of teeth which are borne on the hinge or cardinal margin of 

 the valves. They are very 

 various, but they can usually 

 be divided into the short, 

 stout "cardinal teeth," which 

 are situated under or near 

 the beak, and the ridge-like 

 lateral teeth. The opening 

 of the valves is brought 

 about by an elastic ligament 

 stretched across the hinge 

 from valve to valve, behind 

 the beak, which acts when- 

 ever the tension of the ad- 

 ductor muscles, which close 

 the valves, is relaxed. In 

 many forms, an elastic, com- 

 pressible cartilage, lodged in special grooves or pits, takes the place 

 of the external ligament. The scars marking the attachment of the 

 adductor or closing muscle, or muscles, vary greatly, and are fre- 

 quently preserved in the fossil forms. When two are present they 

 are designated, respectively, as the anterior and posterior adductor 

 scars. The line of attachment of the fleshy mantle which builds the 

 shells, i. e., the palUal line, is often visible. Near the posterior end it 



Fig. 151. Diagrammatic view of left valve of 

 Cytherea: (a. a.) anterior adductor scar; (^. a.) 

 posterior adductor scar; (p. I.) pallial line; { p.s.) 

 pallial sinus; (J.) ligament; it.) teeth; (.m»i.) 

 umbo (aft#r Lang, adapted). 



