PANAMIC-PACIFIC PELECYPODA $1 



the shell when open and through which water is taken in for 

 aeration and feeding, and afterwards expelled. 



Periostracum 



(epidermis). The periostome or the protective external covering of shell 

 composed mostly of organic tissue or chitin with little or no 

 lime added to it, often having a smooth, wrinkled or bristly- 

 surface. Dissolved by alkalines. The periostracum is most heavy 

 in fresh-water mollusks and in some deep-water species as a 

 protection against the corrosive action of acid waters. 



Porcellaneus. A smooth, porcelain-like surface as the interior of some shells 

 (contrasted with nacreous). 



Prodissoconch. The embryonic shell as it emerges from the egg capsule. The 

 shell is usually small, smooth, and is often preserved on the tip 

 of the beaks. 



Prosogyrate. In front or facing forward: the forward coil of the beak so 

 that it faces or points forward or anteriorly. 



Resihfer. The attachment area of the resilium. In the Mactridae, the resihfer 

 is a cup-shaped depression in the middle of the hinge plate. In 

 the Myacidae and Corbulidae, the resihfer is carried on a pro- 

 jecting arm or lamina. 



Resilium. The inner band of the ligament, more or less fibrous in structure 

 and composed of conchiolin charged with lime. Opens the valves 

 through compressional release. 



Tensilium. The outer band of the ligament. Composed of conchiolin but 

 without lime. Usually with a conchoidal fracture, horny. 

 Tensional. 



Umbone. The surface of the valve immediately behind the beak, generally 

 convex. 



Umbonal slope. The line of greatest convexity which can be traced 

 from the beak to the anterior or posterior ventral corners of 

 the valve. 



