734 



ARTHROPODA 



PHYLUM VII 



Family 1. Limnadiidae Baird. 

 Family characters the same as ahove given for the order. 



Estheria Riippel (Figs. 1417, 1418). Shell composed of two tliin rounded valves, 

 united by a straiglit tootliless margin. External surface concentrically ridged or 



FlG 



Estheria minuta Alberti. Lettenkohle Dolo- 

 mite ; Sinsheim, Baden. A, Vi- -B» ^/i- <?> 

 Portion of the exterior, 50/j. 



Fig. 141S. 



Estheria sp. indet. Lower barren 

 Goal Measures ; CarroUton, Ohio. 

 Umbonal portion showing muscular 

 or nuclear node, 13/^. 



striated, and between the ridges are more or less regularly interlacing or branching 

 Striae. The latter character serves to distingiiish this geniis from Posidonomya among 

 Pelecypods. The beaks are not sharply defined, and the primitive portions sometimes 

 bear a strong ocular or muscular node. 



The genus Estheria has numerous fossil representatives, being first met with in the 

 Devonian, and occurring mostly in brackish and shore deposits. It abounds in the 

 productive Goal Measures, in the Permian, Trias (Lettenkohlen mergel) and Wealden, 

 and has been found in the Pleistocene clays of Canada. 



Leaia Jones (Fig. 1419). Carapace marked by one or two diagonal ridges 



Fig. 1419. 

 A, Leaixi leidyi Jones. Goal Measures ; Pottsville, 

 Pennsylvania. B, Uaia Inentschiana Geinitz. Goai 

 Measures; Neunkirchen, near Saarbrücken (after 

 Golden berg). 



Fig. 1420. 



Schizodiscus capsa Glarke. 

 Hamilton ; Centerfield, New 

 York. 2/j. 



whicb ran from the anterior end of the dorsal margin toward the lower margin. 

 Carboniferous ; Europe and North America. 



Estheriella Weiss. Carapace as in Estheria, but with radial riblets crossing the 

 concentric Striae. Permian ; Russia. Buntersandstein ; Saxony 

 _ _ Schizodiscus Clarke (Fig. 1420). Carapace peltate, with a straiglit hinge which 

 18 in the major axis of the shield. Fach valve nearly a semicircle ; surface marked 

 with concentric ridges. Middle Devonian ; New York. 



Lepeditta Matthew. Cambrian ; North America. 



