GLASS I PELECYPODA 427 



forms this area is commonly divided ; a heart-shaped space in front of the 

 beaks, and bounded by a ridge or groove, being known as the lunule ; and a 

 more elongated space extending backward from the beaks being designated the 

 escutcheon. Both areas often have a special sculpture, differing from that of 

 the remainder of the shell. 



Another form of ornamentation is sometimes found on the opposed inner 

 margins of the valves, away from the hinge line, as in Digitaria (Fig. 756), 

 or Transennella ; it probably aids in preventing a lateral displacement of the 

 valves. In general, all ornamentation may be confidently ascribed to a 

 dynamic origin. 



Internal Characters. Soft Parts. — The Pelecypod body is enclosed 

 within two thin, partly fleshy mantle lobes, which are united or continuous 

 below the cardinal margin, and open or partially united at other points on 

 their periphery. Within the mantle lobes are the visceral mass including 

 the internal organs, the gills or ctenidia, the foot and the palpi. When the 

 mantle edges are united so as to form tubes for the entrance and discharge of 

 Avater, such tubes are called siphons. These organs, all of which have been 

 utilised in classification, will be considered separately. 



The mantle is closely applied to the surface of the valves, and is usually 

 attached to them along a line near its periphery. This line is indicated by a 

 continuous scar or impression upon the inner surface of the valves, termed 

 the pallial line. Outside the pallial line a portion of the margin is free and 

 usually thickened. In it are contained the glands which secrete the shell, 

 and also pigment glands ; it is ornamented by papillae, tentacular processes, 

 etc., and is sometimes furnished with visual organs of a primitive sort. 

 Certain archaic forms had no distinct pallial line, the mantle being organi- 

 cally attached over a more or less irregular area. The ends of the pallial 

 line are commonly continuous with the scars of the adductor muscles. 



The majority of Pelecypods have two adductor muscles, and are distinguished 

 accordingly as Dimyarian, or Homomyarian ; in some the anterior muscle is 

 absent or degenerate {Monomyaria) ; and in. others an intermediate condition 

 obtains (Ileteromyaria or Anisomyaria). The number and position of the 

 adductors was formerly accepted as a fundamental feature in classification, 

 although many difficulties were presented by exceptional cases. Recent 

 researches have shown that an absolute foundation for classification cannot 

 be afforded by the number of adductors ; but still, if allowance be made for 

 degeneration caused by inequilaterality, torsion and other causes, the general 

 myarian types harmonise fairly well with the larger divisions based on the 

 totality of characters. 



The visceral mass, as a rule, occupies the upper portion of the shell, 

 and contains the heart, intestinal canal, generative organs, renal and other 

 glands. The rectum usually lies above the posterior adductor, and dis- 

 charges into the anal siphon, when present. The mouth is placed at the 

 forward end of the visceral mass below the anterior adductor, and is commonly 

 furnished on each side with a pair of leaf-like expansions of the integument 

 called palpi, which are ciliate internally, and serve to conduct alimentary 

 matter from the gills to the mouth. Palpi are seldom wanting, and their 

 form and character remains fairly constant throughout a number of groups. 

 The mouth itself is unarmed, and the alimentary canal is more or less bent, 

 usually exhibiting a dilation which is regarded as the stomach. 



