THE BRACHIOPODS. 469 



the hinge will permit. This admirable structure was first 

 pointed out by Dr. Leach, in 1818.* 



The muscular scars and submarginal impression are present 

 in every bivalve shell, but the pallial sinus ,f being marked 

 out by the foot, is of course absent in all the genera which 

 are apodous. There is besides to be observed, in many bi- 

 valves, an impression on the posterior slope, forming on^ or 

 two parallel and adjacent shallow furrows ; this is called the 

 siphonal scar, for it is impressed on the shell by the bran- 

 chial and anal siphons. Hence the presence of this impres- 

 sion is a proof that the animal had these organs, combined or 

 adnate when the furrow is single, and separate when the fur- 

 row is divided by a raised central line. The length of the 

 siphons is indicated by the length of the furrows. 



The inner surface of the bivalves is always smooth, glossy 

 when fresh, frequently nacred or pearly, generally white, 

 but in many tinted with rose-colour, yellow, orange, purple 

 or blue. This colouring is produced, not by glandular secre- 

 tion like the colours of the external surface, but by the con- 

 tact of the inner layers with a similarly coloured viscus of 

 the animal, by whose excretion it is stained. Hence the 

 character furnished by this colouring is of little value in dis- 

 tinguishing genera, or even species. 



The margin of the valves is variously fashioned, but there 

 is no difficulty in understanding the discriminating terms. 

 It is thin and acute, or thickened ; even or undulated or sinu- 

 ate d ; smooth or serrated. If the little denticles which, by 

 their manifold and equal repetition, constitute the serrated 

 character, are made larger and fewer, then a crenulated or 

 crenated margin is the result ; and a toothed margin is one 

 with still larger and few projections. The margins of fresh- 

 water bivalves are never properly toothed or serrated. The 

 only exception is found in Unio sulcatus of Lea, in which 

 the margin approaches to the dentate character. 



In the preceding explanations the Conchifera have been 

 kept exclusively in view, but the Brachiopoda possess like- 

 wise a bivalve shell, and it is now necessary to notice a few 

 terms more peculiar to that singular order. 



The valves of the shell of the Conchifera, when viewed in 

 their relation to the animal in its natural position, are right 

 and left ; but in the Brachiopods one valve is superior and 



* Gray in Zool. Journal, i. 217 220. 



f Mr. Sowerby, in his Genera of Recent and Fossil Shells, always names 

 this the " impressio muscularis pallii," the " muscular impression of the 

 mantle." 



