CLASS I 



PELECYPODA 



429 



two currents. The siphonal septum is frequently carried forward internally, 

 or supplemented by a junction of the gills in such manner as to form a 

 practically continuous partition between the anal and branchial regions 

 within the mantle. The siphons are always contractile, and, except in 

 sedentary burrowers, usually retractile wathin 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, fari 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 



Ftg. 641. 



Fig. 642. 



Lutraria elUptica Roissy. Interior of left valve show- C'rassatellitesplumheus{Chem.). Interior of left 



ing pallial line (p) ; pallial sinus (s) ; anterior (a) and pos- valve showing entire pallial line (m) ; anterior (a) 



terior (»') adductor scars; and resiliifer (/). hv, Length; and posterior (a) adductor scars; and resiliifer 



lit, Height of the sliell. -/-^ natural size. (I). 2/^ natural size. 



regarded as of prime importance, the Pelecypods with a sinus were called 

 Sinupalliafa (Fig. 641), and those without, TntegripaUiafa (Fig. 642). 



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 

 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 Palaeoconcha. 



Since the general form of the Pelecypod depends upon its principal 

 anatomical characters (the size, number and position of the muscles, the 

 presence, size and character of siphons, byssus, etc.), then, to a certain 

 limited extent, especially in the modification of the primitive simple Palaeo- 



