718 LAMELLIBRANCHIATA. 



transverse teeth radiating in a fan-shaped manner from the beak (fig. 

 583, a). The Ordovician genus Actinodonta only differs from Lyrodesma 

 in having a longer hinge-line, and in the fact that the central hinge-teeth 

 are short or obsolete. 



Order V. Submytilacea. 



This order is defined by Fischer as comprising marine or fresh- 

 water Bivalves, in which the mantle-lobes are free, with thickened 

 margins ; the foot may or may not be byssiferous, and there are 

 (except in Midleria only) two adductor muscles. The shell is 

 almost always equivalve, the interior of the valves sometimes 

 nacreous, sometimes not ; the hinge is furnished with teeth, which 

 are differentiated into a " cardinal " and two " lateral " series ; the 

 ligament is external ; and the pallial line is simple. Dr Fischer 

 includes in this order a number of families, of which the most 

 important are the Modiolopsidcz, Trigoniidcz, Unionidce, Ca?'diniidce, 

 Carditidce, Astartidce, and CrassatellidcB. 



Family i. Modiolopsid^e. — In this family the shell is equivalve 

 and very inequilateral, the beaks being placed subterminally towards 

 the anterior end of the shell. There are two adductor impressions, 

 of which the anterior is smaller than the posterior, and the pallial 

 line is entire. The ligament is external, and the hinge may or may 

 not be furnished with teeth. The principal genus of this family is 

 Modiolopsis itself (fig. 590), in which the shell is elongated, re- 



Fig. 590. — Modiolopsis Jiiodiolaris. Ordovician. 



sembling that of a Modiola in shape, with anterior beaks, and 

 having the surface smooth, or marked by fine concentric lines of 

 growth. The posterior end of the shell is broader than the an- 

 terior ; the hinge-line is long and nearly straight, and there is a long 

 ligamental groove extending to the posterior extremity, no teeth 

 being apparently developed. The impression of the anterior ad- 

 ductor is small but deep, that of the posterior adductor being wide 

 and ill-defined. The species of Modiolopsis are Ordovician and 



