42 MOLLUSCA FROM THE CRAG. 
Lima,* Bruguere, 1797. 
Praciostomus. Llwyd. 1698. 
Raputa. Chem. 1784. 
Giaucus and GuraucopErmA. Poli, 1795. 
OsrREA (sp.). Linn. 
Mantettum. Bolten, 1798. 
PeEctEN (sp.). Mont. 
Praciostoma. J. Sow. 1814. 
Guavcton. Oken. 1815. 
Limatuta. S. Wood. 1840. 
Limuta. D’Ord. sec. Gray. 
Gen. Char. Shell ovate, equivalved, generally oblique, inequilateral, and gaping 
at both sides; sometimes closed and equilateral, externally costated or striated, 
radiating from the umbo; often rough and squamous like a file. Hinge area extended 
into auricles, bipartite; cartilage occupying the central or triangular portion ; 
ligament more external and linear. Palleal impression entire, that by the adductor 
muscle large, ovate, and eccentric. 
The animal of this genus has the lobes of the mantle disunited, the margins frmged 
with long tentacular filaments, and without siphonal tubes. A small compressed foot 
furnished with a byssal groove. 
Some species approach very closely to those of the genus Pecten, in being equi- 
lateral, and enclosing the animal within the shells when they are brought together ; 
in others, the shells gape widely, both on the anterior and posterior sides, and the 
animal is too large to be covered by the valves. A subgenus was proposed by myself, 
for those species which are equilateral and closed (under the name Limatula); but 
recent examinations of the animals of both sections are said not to present differences 
sufficient to justify generic separation. They are, therefore, here united. 
The name of Limea was proposed as a genus for those species which are furnished 
with teeth or crenulations upon the hinge margin on each side of the cartilaginous 
pit, and the name Limoarca was also given in consequence to the same section, but 
this character alone, it is to be feared, is not sufficient for generic separation ; 
specimens of Lima subauriculata in my own cabinet, are in like manner supplied with 
minute crenulations. Dr. Loven, however, states the animal of his Zamea Sarsii to 
have the margin of its mantle destitute of tentacular appendages. Species, probably 
belonging to this genus, from the older Secondary Formations figured and described 
under the name of Plagiostoma, have been long known, and were abundant in some of 
the older periods. In those shells the gape or opening appears to have been on the 
rounded or posterior side, on which, in the recent shell, is placed the large adductor 
muscle, while the foot, the organ that secretes the byssus, is on the anterior side, 
which appears to have been capable of being quite closed, the opening, therefore, was 
* Etym. limus crooked, oblique; lima? a file. 
