484 



LAMELLIBRANCHIATA. 



Of the Palaeozoic Bivalves which have been more or less 

 properly referred to the Mytilidce, the following deserve 

 mention. Firstly, we have the important Silurian genus 

 Modiolopsis (fig. 344), in which the shell is equivalve, ve,ry 

 inequilateral, the beaks anterior, and the surface smooth, or 

 marked by fine concentric lines of growth. The shell is 

 thin, and its posterior end is considerably broader than the 



Fig. 344. Modiolopsis modiolaris. Lower Silurian. 



anterior. The hinge is edentulous, and there is a ligamental 

 groove, which begins in front of the beak, and extends to the 

 posterior extremity. 



The genus Orthonota likewise comprises a number of Silu- 

 rian Bivalves, and is also in a somewhat doubtful position. 

 The shell (fig. 345) is elongated, equivalve, very inequilateral, 

 having the beaks placed close to its anterior end. The shell 



is thin, and its margins are par- 

 allel. The hinge seems to be 

 edentulous, and the hinge-line is 

 typically long and straight. In 

 one group, however, of the shells 

 placed here (Orthodesma) the 

 hinge-line is bent or contracted 

 in front of the beaks, and straight 

 behind (fig. 345). 

 We may also, provisionally, place here the genera Myo- 

 concha and Hippopodium, in accordance with the views of 

 Stoliczka; though both these genera have a toothed hinge, 

 and in other respects differ from the Mytilidce and approach 

 the genus Cardita. In Myoconcha the shell is mussel-shaped 

 and thick, with nearly terminal beaks and an external liga- 



Fig. 345 Orthonota (Orthodesma) par- 

 allela. Lower Silurian. 



