752 CLASS xn. 



Note. Genus Necera GRAY, related to the Corbulce should, on account of 

 the ossicle of the ligament, be removed to the first phalanx of this family. 



My a L. (exclusive of several species). Shell subequi valve, 

 transverse, hiant. Left valve with single cardinal tooth large, pro- 

 minent within towards the right valve, and received in the fovea of 

 this. Ligament inserted in tooth and cardinal fovea. Impression of 

 mantle emarginate with a deep sinus posteriorly. 



Sp. Mya truncata L., CHEMN. Conchyl. Tab. I. figs, i, 2, DESH. Conchyl. 

 PL 8, figs, i 4; Mya arenaria L., BASTER NatuurJc. Uitsp. n. bl. 72 76, 

 Tab. vn. figs, i 3, BLAINV. Malacol. PI. 77, fig. i ; on the strand of the 

 N. Sea at Katwijk, Scheveningen, the Swedish coasts, &c. There are 

 some fossil species of Mya in the newer tertiary formations, in which also 

 the two living species mentioned are met with. 



Family XX. Pholadomyacea. Mantle sacciform, produced 

 posteriorly into two elongate conjoined tracheae, perforated ante- 

 riorly by a narrow aperture, emitting the small foot. Shell hiant 

 at both ends. Ligament external. Two muscular impressions 

 remote. 



Pholadomya SowERBY, DESH. Shell thin, transverse, longer 

 posteriorly, obtuse anteriorly. Hinge edentulous, inflected. Im- 

 pression of mantle excised posteriorly by a deep sinus. 



Sp. Pholadomya Candida Sow., DESH. Conchyl. PL 4, figs. 4 6; a W. 

 Indian species still extremely rare in collections. The fossil species of this 

 genus, that occur in almost all geological formations, are numerous, most 

 so in the Lias. To these also DESHAYES refers the genera Lysianassa MUEN- 

 STER, AGASSIZ, and Platymya AGASSIZ. Comp. AGASSIZ Etudes crit. sur 

 les Mollusques, lelivraison, Neufchatel, 1842, &c. 



Ceromya AGASS., DESH. (and Greslyia AGASS.). 



Fossil species. 



Glycimeris LAM., Cystodaria DAUDIN. Shell thick, trans- 

 verse, inequilateral, shorter posteriorly, rounded at both ends. 

 Hinge edentulous. Impression of mantle not excised posteriorly, 

 very distinct, with margin incrassate, undate. 



Sp. Glycimeris Siliqua LAM., Mya siliqua SPENGLER, CHEMN. Conchyl. Tab. 

 198, fig. 1934, BLAINV. Malac. PI. 80, fig. 3; comp. AUDOUIN M6m. sur 

 P Animal de la Olycimere, Ann. des Sc. not. Tom. 28, 1833, pp. 331 343, 

 PL 14, 15. Hitherto the only known species of this genus ; on the coasts 

 of Newfoundland. 



