42 MOLLUSCA FROM THE CRAG. 



LIMA,* Bmguere, 1797. 



PLAGIOSTOMUS. Llwyd. 1698. 



RADULA. Chem. 1784. 



GLAUCUS and GLAUCODERMA. Poll, 1795. 



OSTREA (sp.). Linn. 



MANTELLUM. Bolten, 1798. 



PECTEN (sp.). Mont. 



PLAGIOSTOMA. J. Sow. 1814. 



GLAUCION. OJcen. 1815. 



LIMATULA. 8. Wood. 1840. 



LIMULA. D'Orb. 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 fringed 

 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 Limatuld) ; 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 Limea 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; limat a file. 



