yio 



LAMELLIBRANCHIATA. 



of their valves and the fine striation of the surface. In the Car- 

 boniferous rocks the genus is represented by large forms, and 

 numerous later types are known, the existing species having a wide 

 range in space. 



In the Jurassic and Cretaceous genus Trichites the shell is of large 

 size and great thickness, with a marked fibrous structure, the valves being 

 of unequal size, with twisted beaks and undulated margins. The hinge- 

 line is linear and edentulous, and the impression of the posterior ad- 

 ductor is very long and narrow. Lastly, in the Carboniferous and Per- 

 mian genus Avicidopmna (fig. 579, b), the beaks are placed a little way 

 behind the anterior extremity of the shell, and the surface is nearly 

 smooth, and is marked with concentric striae. 



Family 2. Mytilid^e. — In this family the foot is cylindrical, 

 grooved, and byssiferous, and there are two adductor impressions, 

 that of the anterior adductor being small and terminal in position, 

 while that of the posterior adductor is of large size. The shell is 

 mostly equivalve, wedge-shaped or oval, with anteriorly-placed beaks, 

 not alate, and without a byssal notch ; the byssus passing between 

 the slightly gaping ventral margins of the valves. The cardinal 

 margin is oblique ; the hinge is edentulous ; and the ligament is 

 long, linear, and marginal. The pallial line (except in Dreissenomyd) 

 is entire. The shell is composed of an inner nacreous layer and 

 an outer cellulo-prismatic stratum, with a horny epidermis. The 

 MytilidcE are partly marine and partly inhabitants of brackish or 

 fresh water, and the earliest types of the family appear to be found 

 in the Devonian rocks. 



In the genus Mytilus are the true " Mussels," in which the shell 

 is very inequilateral and wedge-shaped, and the beaks are terminal 



in position. Numerous fossil 

 forms are known, commencing 

 in the Trias, and the genus 

 is largely represented at the 

 present day. The Palaeozoic 

 shells which have been placed 

 under Mytilus probably belong 

 really to other genera. The 

 genus Modiola, comprising the 

 " Horse -mussels," is hardly 

 separable, so far as the shell 

 is concerned, from Mytilus 



Exterior and interior of the left valve proper, the principal differ- 

 rfModtola subcarinata, from the Eocene Tertiary £ being that the shell is 



of the Pans Basin. ° 



rather oblong than cuneiform, 

 and the beaks are rounded, and are placed a little behind the 

 anterior extremity (fig. 580). The genus Modiola appears to be 



