BllACHIOPODA. 



231 



There are no apparent brachial processes in the dorsal valve of Stro- 

 pkomena, and it is possible that the spiral arms may have been supported at 

 some point near the centre of the shell () as in Producta ; S. rhomb oidalis 

 occasionally exhibits traces of spiral arms, in the ventral valve. S, latlssima 

 Bouch. has plain areas, like Calceola. 



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. alternata and rhomboidalis, whilst 

 in S. planumbona and euglypha it be. 

 comes convex; these distinctions are not 

 even sub-generic. 



Fossil, 100 sp. L. Silurian Garb, 

 _ T Fig. 150. Lepttena. f 



N. America, Europe, Thibet. A hlnge areas . V) ventral> B> interior 



S. demissa, Conr. (Stropheodonta, of dorsal valve. ' 



Hall). S. Dutertrii, and several other species have a denticulated hinge-line. 



Sub-genera ? Leptana (part) Dalman. L. transversalis, fig. 150. (Plec- 

 tambonites, Pander.) Valves regularly curved; dorsal concave, thickened, 

 muscular impressions elongated. Fossil, L. Silurian Lias. N. America, 

 Europe. The lias Leptseiias resemble Thecidia internally ; they are free 

 shells, with sometimes a minute foramen at the apex of the triangular delti- 

 dium ; L. liassma, PI. XV. fig. 25. 



Fig, 145. Producta ? Leonhardi, 

 Koninclda, Suess. Producta Leonhardi, AVissm. (P, alpina, Schl.) fig. 145. 

 Trias, St. Cassian. Shell orbicular, concavo-convex, smooth; valves articu- 

 lated ? closely appressed ; ventral valve convex, dorsal concave ; beak in- 

 curved, no hinge-area* nor foramen ? interior of each valve furrowed by two 

 spiral lines of four volutions, directed inwards, and crossing the vascular 

 impressions ; umbo with 3 diverging ridges. The small spiral cavities, once 

 occupied by the arms, and now filled with spar, may be seen in specimens 

 with both valves, by holding them to the light. Mr. Suess of Vienna states 



A, Translucent specimen; B, interior of dorsal valve. 



M 



