750 



LAMELLIBRANCHIATA. 



external. The shell in the forms included here is equivalve, oval 

 or transversely elongated, convex, and thin. The hinge is edentu- 

 lous, and the cardinal border is straight. Fischer regards this family 

 as representing in the Palaeozoic deposits the Arcomyidce of the 

 Secondary period, but the affinities of most of the genera provision- 

 ally associated with Grammy sia must be regarded as very uncertain. 

 In the Silurian and Devonian genus Grammy sia (fig. 636, b) the 

 shell is transversely elongated, equivalve, with the beaks placed very 



Fig. 636. — a, Right side of Palceanatina typa, showing the superior size of the left beak — 

 Devonian (after Hall); B, Grammysia cingulata — Silurian; c, Allorisjna sulcata — Carbon- 

 iferous (after Phillips); d, Leptodomus truncatus— Silurian (after M'Coy). 



far forwards and incurved, a deep " lunule " being present below 

 them. The hinge-line is straight, and the hinge is edentulous ; and 

 a single or double fold extends backwards from the beaks to the 

 middle of the ventral margin. 



The following Palaeozoic genera agree with Grammysia in more or 

 fewer characters, and may be provisionally associated with it, though 

 they diverge in important respects from the above general definition of 

 the family Grammysiidce. In Cardiomorpha (Silurian to Carboniferous) 

 the shell is shaped like that of Isocardia, with approximated, almost ter- 

 minal beaks, and a simple pallial line. The hinge is toothless, and there 

 is an elongated groove for the external ligament. In the genus Leptodo- 

 mus, with a similar geological range to the preceding, the shell (fig. 636, d) 

 is thin and elongated, with tumid incurved beaks, a well-marked posterior 

 slope, and a deep " lunule." The hinge is toothless, and the surface is 

 marked with concentric ridges which split anteriorly. The widely distrib- 

 uted Carboniferous genus Edmondia has "a transversely -oval, equi- 

 valve, edentulous shell, with an internal lamellar cartilage-support. The 

 dorsal margins are erect and simple, and the pallial line is entire " (R. 

 Etheridge, jun.) In the genus Sanguinolites, principally if not exclu- 

 sively Carboniferous in its range, the shell is " transversely oblong and 



