ORCICULA. 



Shell ineqiiivalve, siiborbiciilar, generally rather 

 irregular^ and conforming- itself to the inequalities of the 

 substance to which it is attached. Upper valve com- 

 pressedly patelliform, its vertex posterior: lower valve 

 attached, flat, and thin. Four muscular impressions in 

 each valve, two of which are large, approximate and near 

 the center, and two smaller, more distant and near the 

 posterior margin : those in the lower valve are seldom so 

 distinct as those of the upper. A fissure for the passage 

 of the substance by which the lower valve is attached is 

 seen near its center^ and a rather obtuse shelly process is 

 placed at the inner extremity of this fissure. There are 

 no hinge teeth, nor is there any ligament. 



The animal of the Orbicula has two fringed arms, 

 like those of Crania, Terebratula, &c. it therefore belongs 

 to Lamarck's family of Brachiopoda; we have never seen 

 it but in a dry state, but even then its arms are quite dis- 

 tinct, and their fringe sometimes extends beyond the 

 closed edges of the shell. 



Besides the two recent species figured in the "Linn. 

 Trans." we have never seen any other, unless indeed some 

 small and very regular specimens which we have seen 

 attached to a common Pearl Oyster, and of which we 

 have represented one in our present plate, belong to a 

 distinct species: this is a point we cannot decide, and 

 must therefore leave it until we have an opportunity of 

 examining more specimens. Nor have we any positive 

 evidence of the existence of fossil species, though the 

 Patellae latissima et laevis of " Sowerby's Mineral Con- 

 chology," t. 139, may possibly be the upper valves of 

 Orbiculas. 



Fig. 1 . Orbicula Icevis^ upon a rolled flint pebble, and accompanied by Ae 

 roots of an Isis. 



2. Under side of the attached valve showing the fissure. 



3. Orbicula Norvegica^ outside of the upper valve. 



4. Inside of the same. 



5. Inside of the lower valve of the same. 



