/ 



PIIYLLOPODA. 



511 



which in some respects constitutes a connecting-link between the 

 Phyllopods and the Ostracodes. The body in Estheria is enclosed 

 in a bivalve carapace, and the feet are foliaceous. The valves of 

 the carapace have a well-marked beak or " umbo," and are hinged 

 to one another along a dorsal line. From these circumstances, and 

 from their being marked with numerous concentric lines of growth, 

 the carapace-valves of Estheria very closely resemble the shells of 

 certain Bivalve Molluscs {Posidonia and Posidonomya), for which 

 they have often been mistaken. The valves are usually sub- 

 triangular, ovate, or sub-quadrate in form, and they possess a horny 

 texture. 



The living Estheria are, without exception, inhabitants of fresh, 

 or, rarely, brackish water ; and no one of the recent twenty-four 

 species has been detected in the sea. This would afford a strong 



Fig. 363 — a, Carapace of Estheria tn>ata, magnified six diameters — Trias ; b, Carapace of 

 Leaia LeiJyi, magnified five diameters — Lower Carboniferous. (After Rupert Jones.) 



presumption that the deposits in which Estheria occur were laid 

 down in fresh or brackish water ; but such fossils not uncommonly 

 occur in conjunction with undoubted marine remains. They 

 appear, on the whole, to occur most frequently in those accumu- 

 lations that " have been decidedly the result of brackish- water 

 inundations, and of more permanent lagoons " (Jones). Fossil 

 Estheria occur in the Devonian, Carboniferous, Permian, Triassic, 

 Jurassic, Cretaceous, and some Tertiary deposits ; but they appear 

 to have attained their maximum development towards the close of 

 the Triassic period. The allied genus Schizodiscus is Devonian, and 

 Estheriella is found in the Trias. 



The genus Leaia (fig. 363, b) is very nearly allied to Estheria, 

 and comprises small Bivalved Crustaceans, with " dark, horny, sub- 

 quadrate valves, obliquely ridged from umbo to angles, and orna- 

 mented with distinct lines of growth parallel with the border" 

 (Jones). Leaia is a very widely distributed genus, but all the 

 known species belong to either the Carboniferous or Permian rocks. 

 It has been suggested, however, that the obscure Ordovician genera 

 Myocaris and Ribeiria are allied to Leaia. 



