BRACHIPODA. 197 



not so great as in species of that section. Tliese features, however, are 

 regarded as of less importance, but on the basis of internal structure may be 

 included such forms as Orthis porcata, McCoy, in which the brachial valve is very 

 convex, and Orthis retrorsa, Salter, where the retrorsion of the beak is carried 

 to a greater extreme than in any other species of Orthis. 



In the interior of the pedicle-valve there is a large quadrate muscular scar 

 with a tendency to bilobation by the lateral extension of the diductor impres- 

 sions. The subdivision of this area into separate scars is frequently very dis- 

 tinct. In the brachial valve the cardinal process is thickened and erect at its 

 posterior extremity, being distinctly crenulate on the summit and posterior 

 face. The surface striae are tubulose and the shell-structure fibrous-im- 

 punctate. 



While the muscular scars in this group are somewhat similar to those of 

 Group III, and the external features like those of Group V, the combination 

 here described is persistent, though represented by few species. To those 

 above mentioned may be added 0. reversa, Salter, from the Lower Llandovery, 

 and probably several others from the Silurian of Great Britain. 



The type appears to be limited to the faunas of the Lower Silurian. 



There is a small group of early species, agreeing with Orthis subquadrata in 

 the reversion of the valves and the form of the muscular scars and cardinal 

 process, but characterized by the existence of a more or less completely devel- 

 oped convex deltidium. To this group belong the Strophomena defleda and S. 

 recta of Conrad, and also a new form here described as Orthis loricula, all from 

 the horizon of the Trenton limestone. These shells have an elongate hinge- 

 line, and their external expression is much like that of Strophomena, a resem- 

 blance which is increased by the presence of the convex deltidium suggesting 

 a divergence at this point from Orthis toward Strophomena. 



