ERACniOPODA. 



381 



valve "witli a bi-lobed cardinal proceps, between the dental 

 sockets, and four depressions for tlie adductor muscles. 



Ventral valve. Fig. 184. Dorsal valve. 



Inteiior of 5. rhomboid ahs, var. analoga, Carb. limestone (after King), 

 e, foramen ; t, teeth ; o, ovarian spaces ; b, bracliial 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 {b) as 

 in Frodudus ; S. rhoniboidalis occasionally exhibits traces of 

 spiral arms, in the ventral valve. 

 S. latissima, Bouch., has plain 

 areas, like Calceola. 



The valves of the Strophomenas 

 are nearly flat untH they approach 

 their full growth, they then bend 

 abruptly to one side ; the dorsal 

 valve becomes concave in S. cdter- 

 nata and rhomhoidah's, whilst in 

 S. planumhona and euqh/pha it 



, ^ ,-, n- ,■ Fig. 18.3. LcptcEiia, f. 



becomes convex; these distmc- a, hinge-areas .- v, ventral; b, interior 



tions are not even sub-generic. of dorsal valve. 



i^osstV, 129 species. Lower Silurian — Carb. North America, 

 Europe, Thibet. 



S. demissa, Conr. (Stropheodonta, Hall), S. JDutertrn, and 

 several other species have a denticulated hinge-line. 



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

 Fig. 185. (Plectambonites, Pander.) Valves 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 XV., Fig. 25. 



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

 Schl.), Fig. 186. Trias, St. Cassian. /SAe// orbicular, concavo- 

 convex, smooth ; valves articulated ? closely apprcssed ; ventral 

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



