BEACHIOFODA. 



381 



valve "witli a bi-lobed cardinal process, between tbe dental 

 sockets, and four depressions for the adductor muscles. 



Ventral valve. 



Dorsal valve. 



Fig. 184. 



Interior of S. rhomboidahs, var, analoga, Carb. limestone (after King). 

 e, foramen ; t, teeth ; o, ovarian spaces ; b, brachial pits ? 



There are no apparent brachial processes in the dorsal valve 

 of Strophomena, and it is possible that the spiral arms may have 

 been supported at some point near the centre of the shell (&) as 

 in Frodudus ; S. rhomboidalis occasionally exhibits traces of 

 spiral arms, in the ventral valve. 

 8. latissima, Bouch., has plain 

 areas, like Oalceola. 



The valves of the Strophomenas 

 are nearly flat until they approach 

 their full growth, they then bend 

 abruptly to one side ; the dorsal 

 valve becomes concave in >S^. altei-- 

 nata and rhomhoidalis, whilst in 

 S. planumhona and euglypha it 

 becomes convex; these distinc- 

 tions are not even sub-generic. 



Fossil, 129 species. 

 Europe, Thibet. 



S. demissa, Conr. (Stropheodonta, Hall), 8. Dutertrii, and 

 several other species have a denticulated hinge-line. 



Sub-genera ? Leptoena (part), Dalman. L. transversalis, 

 Pig. 185. (Plectambonites, Pander.) Yalves regularly curved; 

 dorsal concave, thickened, muscular impressions elongated. 

 Fossil, 41 species. Lower Silurian — Lias. North America and 

 Europe. The lias Leptcenas resemble Thecidia internally ; they 

 are free shells, with sometimes a minute foramen at the apex of 

 the triangular deltidium ; L. liassina, PL XY., Fig. 25. 



Koninckia, Suess. Producta Leonhardi, Wissm. (P. alpina, 

 Schl.), Pig. 186. Trias, St. Cassian. >S/ie/^ orbicular, concavo- 

 convex, smooth ; valves articulated ? closely appressed ; ventral 

 valve convex, dorsal concave ; beak incurved, no hinge-area 



Fig. 185. Leptana, |-. 

 A, hinge-areas ; v, ventral ; b, interior 

 of dorsal valve. 



Lower Silurian— Carb. ISTorth America, 



