426 MOLLUSCA phylum vi 



such as Ostrea, Pedalion, etc., the two axes are at a considerable angle with each 

 other. 



The dental armature is usually situated on the dorsal margin, which for 

 this reason is called the cardinal margin. It comprises the teeth, or project- 

 ing processes and sockets, usually alternating in the single valve, and opposite 

 with respect to both valves. In the more modern and perfected types, the 

 cardinal margin is reinforced by a vertical deposit of shell in the form of a 

 lamina called the hinge plate, upon which the teeth are set. Above the hinge 

 plate in each valve rise the beaks or umbones, which are usually curved 

 toward the antei'ior end of the shell (prosogyrate), but are sometimes directed 

 backward (opisthogyrate) or outward (spirogyrate). 



According to the ordinary terminology, the height of a Pelecypod is 

 measured on a vertical line from the beaks to the ventral margin ; the length 

 corresponds to the greatest distance between the margins parallel with the 

 antero-posterior axis above defined ; and the thickness, or diameter, is measured 

 by a line at right angles to the vertical plane descending from the cardinal 

 axis (Fig. 641). AVhen the shell is placed with the oral end anterior, the 

 valves are termed right and left respectively, as viewed from above the 

 articulus. The portion of the shell anterior to the beaks is usually shorter 

 than that behind them, except in such forms as Donax or Niicula. 



Viewed laterally, most Pelecypod valves may be divided into regions, 

 corresponding in the main to the disposition of the internal organs. The oral 

 area extends from the anterior end of the cardinal line to the anterior side of 

 the pedal area. The latter is often marked by a swelling of the valves, and 

 sometimes by a sinus (Pholas) ; it extends backward to a point where the 

 branchial crest, radiating from the beaks, forms the anterior boundary of the 

 si-phonal area. The dorsal or posterior limit of the siphonal area is marked by 

 an angle in the incremental lines ; and above this, extending to the posterior 

 end of the cardinal line, is the intestinal area. In the alate forms, like Pteria, 

 the wings usually called anterior and posterior are really, with reference to 

 the antero-posterior axis of the animal, dorsal and ventral. 



In certain borers, the siphons are greatly produced beyond the valves, and 

 a calcareous tube is secreted, lining the burrow ; the valves, situated at the 

 anterior end of the boring, either lie free, or are partially or wholly fused 

 with the tube. In the Pholadidae the naked portions of the animal between 

 the edges of the valves are often protected by additional shelly pieces, which 

 are organically separate from the valves ; and some burrowers have the free 

 ends of the siphons protected by leathery or calcareous shields. In the 

 Teredinidae these shields are specially modified to protect the entrance of the 

 burrow, and are called " pallets." 



Ornamentation. — The external ornamentation of the valves is always a 

 conspicuous character. It comprises (beside the concentric or .incremental 

 lines which indicate the successive additions to the shell margin, and are 

 believed to coincide with resting stages during the process of growth) radial 

 or concentric striae, ridges, ribs, folds, nodes, spines or foliaceous processes. 

 These are supposed to arise from temporary or permanent modifications of 

 the mantle margins, such as papillae, minute tentacular or proliferate 

 processes. 



Above the hinge line, in archaic types, is an area often set off" by an 

 impressed line and called the cardinal area. In the more perfected modern 



