580 Anatomy of the Lamellibranchiata. 



mouth a rudiment of the semilunar lip of the Orbicula. The 

 intestinal canal is equally short, much shorter than in any 

 other lamellibranchiate bivalve. The ovaries are ramified in 

 the same way in the mantle. In the Anemia, however, the 

 body is, as it were, awry, having a tendency to assume the or- 

 dinary position in the shells of the Lamellibranchiata ; the 

 mouth is oblique, and the intestine altered in direction, from 

 the same displacement. The left ventricle of the. Orbicula 

 is, in the Anomia, dragged to its fellow, forming a single 

 heart upon the intestine. The four bronchia of the Anomia, 

 and the two tentacles which are undeveloped, appear to go, 

 in the Orbicula, to form the four upper and two lower sets of 

 branchial vessels. The Anomia has the muscular foot of the 

 Orbicula, though little developed, the animal adhering not by 

 it, but by a calcareous operculum. The muscular system is 

 very different from that of the other Lamellibranchiata, though 

 it is in some respects analogous. 



The fossil Spharulites were probably allied to the Anomia. 

 An acquaintance with the anatomy of some unknown living 

 animals, as Thecidea, Crania, Placuna, and of the different 

 species of Terebratula, Anomia, &c, would probably enable 

 us to form a more correct idea of many of the fossil Acephala, 

 so common in various strata. 



In arranging the Lamellibranchiata, a prior division into 

 Monomyaria and JJimyaria, as was done by Lamarck, may 

 be made. The former have but one muscle to close the valves ; 

 the mantle unfurnished with separate orifices or tubes, and 

 open in its circumference, except on the dorsal side ; the 

 valves are commonly orbicular, or if lengthened, are so in the 

 vertical direction, and are often eared at the hinge ; there is 

 generally a degree of obliquity in the animal, and an irregula- 

 rity of the valves arising from a tendency to assume the position 

 which the brachyopodous animal has in its shell. Notwith- 

 standing this last circumstance, however, we can more cor- 

 rectly consider the valves as lateral, than as superior and in- 

 ferior ; though one, from its adherence to foreign bodies, may 

 be inferior when in the natural position. The most convex 

 valve is commonly the adherent one ; in the oyster this is the 

 left ; in the Spondylus, Pedum, &c, it is the right. The foot 

 in the Monomyaria is little developed, not being used for an 

 organ of movement. The Dimyaria have two adductor mus- 

 cles of the valves, an anterior one being added to that of the 

 Monomyaria ; the form of the valves is commonly more ho- 

 rizontally lengthened ; the shell is generally equivalve, and, 

 when at all unsymmetrical, is only so from the nature of the 

 hinge requiring a greater convexity of one valve than of the 



