conchifera. 



Family Y. — Trigoniad^.* 



? IscHYMisrA, Billings, 1866. 



Type, I. Winchelli, Billings. 



Bhell eqnivalve, inequilateral, two strong ridges radiating 

 from the beak in the interior of each yalve. 



Fossil, 2 species. L. and M. Silurian. Anticosti. 



Family YI. — TJNiONiDiE.i- 

 Ajsttheacosia, King, 1856 (see p. 470). 



Etymology, anthrax, carbon, in allusion to the carbonaceous' 

 deposits in which the genus is usually found. 



Type, A. Beaniana, King. Coal Measures, Newcastle. 



Shell equiyalve, inequilateral. Teeth one in each valve below 

 the umbone, rather low and massive ; crown of tooth of right 

 valve excavated anteriorly and ridged posteriorly; crown of 

 tooth of left valve ridged anteriorly and sloped posteriorly. 

 Umbonal Ugamental fulcra, each a furrow excavated in the 

 hinge-plate, between the umbone and tooth. Scars of the an- 

 terior set of pedal muscles, situated above the anterior adductor 

 muscular impressions, 



Anthracosia differs from Unio, to which genus the majority of 

 the Unioniform shells have been referred, in its simpler dental 

 system and in the absence of supplementary pedal muscles. It 

 has no relation to Cardinia, in which genus others of the 

 Unioniform species have been placed ; whilst other members of 

 genus possessing the outward appearance oY certain aviculoid 

 forms of Modiola have been ranged in the genus Myalina. 



Distribution, 61 species. Devonian — Carboniferous. West- 

 phalia, Saxony, Eussia, Belgium, Great Britain, N. America. 



Caebonicola, M'Coy, 1855 (see p. 470). 



Synonym, Prisconaia, Conrad, 1867. 



Example, Unio acutus, Sow. 



Cardinal tooth in right valve diverging obliquely towards the 

 posterior side ; lateral 1-1, long and lamellar; no lunate im- 

 pression on the ventral margin of the anterior adductor as in 

 Unio. 



This genus is related to Anthracosia^ but differs in having 

 lateral teeth. 



Distribution, about 20 species. Coal Measures. Europe, 

 United States. 



* See p. 430. t See p. 432. 



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