352 MOLLUSCA SUB-KINGDOM VI 
practically continuous partition between the anal and branchial regions 
within the mantle. The siphons are always contractile, and, except in 
sedentary burrowers, usually retractile within the shell. 
The siphons, being a local modification of the mantle margin, receive their 
musculation from the same source. In general, the muscles have spread 
inward, pari passu with the increase in length of the organ to be retracted, 
and their insertion on the valve leaves an angular scar called the pallial sinus, 
which is an important aid in classification of the minor groups. It has some- 
times been assumed that the absence of this sinus was evidence of the 
asiphonate character of the species, but the example of Lucina, Cuspidaria, 
and several other siphonate forms which have no pallial sinus show that this 
is not necessarily true. Formerly, when the character of the pallial line was 
regarded as of prime importance, the Pelecypods with a sinus were called 
Sinupalliata (Fig. 592), and those without, Integripalliata (Fig. 593). 
The Hinge.—The origin both of the hinge structure and the ornamentation 
of the shell can be perhaps best understood by a consideration of what is 

Fic. 592. Fic. 593. 
Lutraria elliptica, Roissy. Interior of left valve show- Crassatellites plumbea, Chem. sp. Interior of 
ing pallial line (p); pallial sinus (s); anterior (a) and left valve showing entire pallial line (m); anterior 
posterior (a) adductor scars; and resiliifer (/). hv, (a) and posterior (a’) adductor sears ; and resiliifer 
Length ; wi, Height of the shell. 2/3 natural size. (1). 2/3 natural size. 
known regarding the archetype of the class, and by noticing the changes that 
have since been introduced. The original protopelecypod was small, thin, 
symmetrical, sub-circular or oval, with a short external ligament equally dis- 
posed on each side of the beak along the hinge line. The mantle was not 
uniformly attached to the shell along a pallial line, as in modern Pelecypods, 
but adhered more or less irregularly and was not provided with extrusile 
siphonal tubes. The adductor muscles were sub- equal, symmetrical, and 
situated high up in the valves. The surface of the valves was smooth, or 
(probably in connection with the development of tactile papillae on the mantle 
edge) radiately ribbed. These conclusions are justified not only by inference 
and by recent investigations on the morphology of the prodissoconch, but by 
the characters of the most archaic Pelecypods, summarised by Neumayr under 
the name of Palacoconcha. 
Since the general form of the Peleeypod depends upon its principal 
anatomical characters (the size, number, and position of the muscles, the 
presence, size, and character of siphons, byssus, ete.), then, to a certain 
limited extent, especially in the modification of the primitive simple Palaeo- 
conchs, it is plain that the differences of form would march with the respective 
anatomical differences. For example, those forms which retained the simple 
