350 MOLLUSCA SUB-KINGDOM VI 
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 Woodia (Fig. 707), 
or Transenella; 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. Soff 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 
water, 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 (Heteromyaria 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 re- 
searches 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. 
The Foot——From the ventral surface of the visceral mass projects an 
extensile muscular organ, known as the foot, which is capable of being pro- 
