LAMELLIBEANCHIATA. 487 



Olasslflcatlon of paleozoic Lamelllbrancliiata.] 



Family PHOLA-DELLID^, Miller. 



Geneka : Allorisma, King; Chcenomya, Meek and Worthen; Cimitaria, Hall; Pholadella, Hall; Physe- 

 tomya, Ulrich; Rhytimya, Ulrich. 



Family GRAMMYSIID^, Hall. 



Genera : Cuneamya, Usdl and Whitfield; Grammynia, DeVerneuil; ? Lepfodomus, McCoy; ? Sedgwickia, 

 McCoy; f Sphenolium, Miller; Saffordia, Ulrich. •» 



Family ASTARTTD^. Gray. 



Genus : Astartella, Hall. 



Family MEGALODONTID^, Zittel. 



Genera: Megalodon, Sowerby; Megalomus, Hall; ? Flethocardia, Ulrich. 



Family LUCINID^, Deshayes. 



Genus : Paracyclas, Hall. 



Family CYCLOCONCHID^, (Provisional). 



Genera : Cycloconeha, Miller; ? Anodontopsis, McCoy. 



Family CONOCARDIID^, Miller. 



Genera: Oonocardium, Brov/n; ? Eopteria, Billings; ^!te7iasma, Billings. 



Family CARDIOMORPHID./E, Miller. 



Genera: Oardiomorpha, Konmi^k; Edmondia, lioni nek; Kutkydesma, lia,ll; Protomya,TI All. 



Family SOLENIDiE, Adams. 



Genus: Solenopsis, McCoy; Palceosolen, Hall; ? Orthonota, Conrad. 



Family PAL^ANATINID^, Miller. 



Genera: Ilionia, Billings; Palceanatina, Hall; Prorhynchus, Hall. 



Family PROTHYRID^, Hall. 

 Genus : Prothyris, Meek. 



The literature pertaining to Lower Silurian Lamellibranchiata is not only- 

 meager but, in great part, unreliable. The principal cause for the latter is to be 

 found in the want of experience of the authors, who, failing to understand the effects 

 of pressure to which a large proportion of the shells have been subjected, have thrown 

 together as identical widely different forms, and oftener, perhaps, distinguished the 

 distorted specimens from those which have more nearly retained the normal form. 

 The illustrations also are too often, if not entirely worthless, misleading. Here, more 

 than in any other part of the study, the greatest care and experience are required. 

 Entire and undistorted specimens are not by any means the rule, so that slight 



