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LAMELLIBRANCHIATA. 487 
Olassification of paleozoic Lamellibranchiata. ] 
- Family PHOLADELLIDAS, Miller. 
GENERA: Allorisma, King; Cheenomya, Meek and Worthen; Cimitaria, Wall; Pholadella, Hall; Physe- 
tomya, Ulrich; Rhytimya, Ulrich. 
Family GRAMMYSIIDAS, Hall. 
GENERA: Cuneamya, Hall and Whitfield; Grammysia, De Verneuil; ? Leptodomus, McCoy; ? Sedgwickia, 
McCoy; ? Sphenolium, Miller; Saffordia, Ulrich. 
Family ASTARTIDAX. Gray. 
GENUS: Astartella, Hall. = 
Family MEGALODONTID A, Zittel. 
GENERA: Megalodon, Sowerby; Megalomus, Hall; ? Plethocardia, Ulrich. 
Family LUCINIDAS, Deshayes. 
GENUS: Paracyclas, Hall. 
Family CYCLOCONCHIDAS, (Provisional). 
GENERA: Oycloconcha, Miller; ? Anodontopsis, McCoy. 
Family CONOCARDIIDAS, Miller. 
GENERA: Conocardium, Brown; ? Kopteria, Billings; Huchasma, Billings. 
Family CARDIOMORPHIDAS, Miller. 
GENERA: Cardiomorpha, Koninck; Hdmondia, Koninck; Huthydesma, Hall; Protomya, Hall. 
Family SOLENIDA, Adams. 
Genus: Solenopsis, McCoy; Palwosolen, Hall; ? Orihoncta, Conrad. 
Family PALAXSANATINID AS, Miller. 
GENERA: Jlionia, Billings; Paleanatina, Hall; Prorhynchus, Hall. 
Family PROTHYRIDA, 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 
