THE LAMELLIBRANCHTA 91 



III. Burrowing Branch. 



DESMODONTA. 



Usually equivalve, tending to an oblong shape ; often 

 gaping ; isomyarian ; internally nacreous or porcellanous. 

 Hinge-line simple, usually one cardinal tooth in each 

 valve ; ligament opisthodetic. With long siphons ; 

 usually sinu-palliate. Ornament concentric-striate, rarely 

 radial. 



A number of Palaeozoic genera belong to this branch, 

 but, owing to imperfect preservation, are not capable of 

 being further classified : such are Orthonota (Fig. 25, h, 

 Sil. ), Grammy sia (Sil.-Dev.), and Edmondia (Carb.- 

 Permian). The Mesozoic and later forms can be more 

 definitely grouped. 



i. Anatinacea. Equi- or inequivalve ; hinge with 

 one cardinal tooth in each valve, opisthodetic ligament, 

 with or without resilium ; internally nacreous (with some 

 exceptions) ; sinu-palliate. 



Pleuromya (Trias.-Lower Cret.) is elongate-oval, with 

 fairly strong concentric ornament, and the right valve 

 slightly overlaps the left along the hinge line. Pholadomya 

 (Jur.-Rec.) has been already described; it is almost 

 the only Anatinacean with radial ornament, and is divided 

 into a number of sub-genera according to shape and 

 ornament. Thracia (Trias.-Rec.) is inequivalve, the right 

 valve being the larger; Panopea (Cret.-Rec.) has a thick 

 shell, gaping at both ends ; Saxicava (Cainozoic and 

 Recent) is a rock-borer, with the pallial line represented 

 by a series of disconnected marks. The last two are 

 non-nacreous. 



2. Myacea. Equi- or inequivalve ; ligament in- 

 ternal, on a horizontal resiliophore ; internally porcel- 

 lanous ; sinu-palliate. 



