36 PALAEONTOLOGY OF THE UPPER MISSOURI. 



FAMILY 



Shell thin, often inequivalve, inequilateral, more or less gaping pos- 

 teriorly, pearly within. Surface, in well preserved specimens, generally 

 granulose, concentrically or radiately striate or costate. Hinge teeth 

 usually rudimentary or obsolete ; ligament external, thin ; cartilage occu- 

 pying an internal pit or cavity under the beak of each valve, and usually 

 provided with a free ossicle. Beaks sometimes fissured. Muscular im- 

 pressions faint ; pallial line generally sinuous. 



Animal with long, more or less separated siphons, which are fringed 

 at the extremities ; mantle with united margins, provided with a valve- 

 like opening under the siphons ; gills single on each side, pinnate outer 

 laminae prolonged dorsally beyond the line of attachment. 



A number of fossil genera appear to belong to this family, though their affinities 

 have not been very clearly determined. It is possible some of those mentioned 

 below may belong to one or more distinct families ; but until their relations can 

 be made out more satisfactorily, from the study and examination of the hinge and 

 interior of a larger number of species, we prefer to place them here. 



The existing genera, properly included in this group, are Anatina, Peri/ploma, 

 TJiracia, Lyonsia, Mytilimeria, Poromya, Myodora, Pandorella, Ccelodon, Pandora, 

 Clidiophora, Tfeora, Necera? Tyleria, and Pholadomya. 



The extinct groups, apparently belonging here, are Margaritaria, Cercomya, 

 Anatimya, Allorisma, Myacites, Homomya, Anthracomya ? Chcenomya, Platymya, 

 Arcomya, Mactromya, Goniomya, Gresslya, Cardiomorpha, Ceromya, /Sedgwickia, 

 Sanguinolites, and probably Cleobis and some of the species included in the genus 

 Orthonota. 



Genus ALLOEISMA, KING. 



Synon. Sanyuinolaria (gibbosa), SOWEKBT, Min. Conch. VI, 1814, 92. 



Myacites (sp.), SCHLOT. f Petrefact. 1820, 176. 



Hiatella (sulcata), FLEMING, Brit. An. 313. 



Pholadomya (elongata), MORTON, Am. Jour. Soi. XXIX, 1836. 



Sanguinolites (part), McCoy, Carb. Foss. Ireland, 1844, 47. 



Allorisma, KIKO, Ann. Mag. Nat. Hist. XIV, 1844, 315 ; Mon. Permian Foss. England, 1850, 196. 

 Etym. ixWoc, variable ; t^i^/jut, support. 

 Examp. Hiatella sulcata, FLEMING.' 



Shell equivalve, inequilateral, elongate, thin ; anterior side short ; posterior side 

 long and somewhat gaping at the extremity ; beaks depressed, anterior. Surface 

 minutely granulose, and ornamented with more or less distinct concentric ridges 



1 As first defined by Prof. King, this genus was made to also include species belonging to the gonus 

 Edmondia. We observe that he remarks in a foot-note to page 196 of his Monograph of the Permian 

 Fossils of England, published in 1850, that he avails himself of that opportunity to name Hialella 

 sulcata as the type of this genus, instead of Allorisma regularis of Murchison, Verneuile & Keyser- 

 ling's work on the Fossils of Russia ; because he thinks the latter more probably an Edmondia. 



