258 ARCIDJS. 



MEGAMBONiA, Hall, 1859. Shell ventricose, broad-ovate or 

 suborbicular ; anterior end with a ventricose wing or extension, 

 separated from the body of the shell by a deep curved sinus ; 

 anterior hinge-teeth unknown ; lateral teeth two to three, short, 

 small and oblique ; ligamental area narrow or obsolete ; anterior 

 muscular scar deep, circular, and having a small pedal scar upon 

 its upper margin ; posterior scar larger, faintly impressed; pallial 

 line simple. G. aviculoidea, Hall (cxxix, 9). 



Adranarta, Munier-Chalmas, 1876. 

 -Syn. — Siliquarca, Pseudarca, Tromelin and Lebesc. 

 Distr. — 2 sp. Palaeozoic ; France. A. Tromelini, M.-C. 

 Shell equivalve, recalling Cultellus by its general form ; hinge 

 with a series of oblique linear, nearly parallel teeth, the anterior 

 ones narrower and more divergent, the posterior more numerous ; 

 a rib within each valve passes in front of the anterior muscular 

 impression. 



Cardiola, Broderip, 1834. 



Distr. — It sp. U. Sil. — Devon. ; Eur., U. S. G. cornucopise, 

 (cxvi, 94). 



Shell somewhat inequilateral or subequilateral, roundly ovate, 

 with the beaks incurved anteriorly and with a rather large liga- 

 mental area between both ; hinge-line slightly curved with 

 numerous oblique pliciform teeth, being apparently present on 

 both sides of the beak ; surface generally radiately striated or 

 ribbed. 



Lyrodesma, Conrad, 1841. 



Syn. — Actinodonta, Phil. 



Distr. — Fossil, 4 sp. Lower Silurian ; Canada, United States, 

 Britain. L. pulchella, Hall (cxxvii, 83). 



Shell Trigonia-shaped, rather elongated, with a striated poste- 

 rior area ; hinge with several (5-9) radiating teeth, striated 

 across ; ligament external. 



Lyrodesma appears to connect the Pectunculus with the 

 Arcinse. 



Pectunculus, Lam. 



Syn. — Axinsea, Poll, 1791. Tuceta, Bolten. 



Distr. — 58 sp. West Indies, Britain, India, New Zealand, 

 West America; ranging from 8 to 60, rarely 120 fathoms. 

 Fossil, 80 sp. Neocomian — ; Eui'ope, United States, South 

 India. P. Delesserti, Reeve (cxxvii, 53). 



Shell orbicular, nearly equilatei*al, smooth or radiately striated ; 

 umbones central, divided by a striated ligamental area ; hinge 

 with a semicircular row of transverse teeth ; adductors sub- 

 equal ; pallial line simple ; margins crenated inside. 



Animal with a large crescent-shaped foot, margins of the sole 



