298 POPULAR CONCHOLOGY. 



Muscular Impressions. The indented marks in acepha- 

 lous Bivalves, which indicate the insertion of the mus- 

 cles, by which the animal is attached to its shell. Of 

 these there are three principal sorts : lateral, central, 

 and pallial. 1. The lateral impressions indicate the 

 animal to have two adductor muscles, as in Unio, 

 Cardium. 2. A central impression shows the pre- 

 sence of one adductor muscle, which is generally in 

 the middle of the shell, as the Oyster. All these 

 shells are, however, furnished with a third or pallial 

 impression, which is indicated by a depressed line 

 running parallel with the ventral margin. (See Ad- 

 ductor Muscle.} 



Obsolete. This term implies a faint indication of par- 

 ticular characters. Thus, very slight or partially in- 

 distinct striae upon a shell are called obsolete striae, 

 not because they are worn off by accident, but because 

 they are only rudimentary, or very slightly developed. 



Operculum. A hard shelly or horny lid, which covers 

 the aperture of many spiral shells, and is attached to 

 the foot of the animal. (See Epiphragma.) 



Orbicidar. Quite round. 



Ovate. Egg-shaped. 



Papyraceous. Excessively thin, like paper. 



Peduncle. A stalk, or support. 



Pelagic. Belonging to, or found in, deep seas. 



Perforated. Having holes, as if bored by a sharp instru- 

 ment, as the Haliotis. 



Phosphorescent. Shining in the dark, like the glow- 

 worm. 



Plaits. Folds, or raised lines on the Columella. 

 Polythalamous. (See Chambered and Monothalamous.) 

 Posterior. The side of the shell, in Bivalves, opposite to 



