350 ARTHROPODA. 



its two halves are united in front at an umbonal point, 

 and are only imperfectly divided behind by a straight ridge 

 or fold. 



More widely distributed than any of the preceding is the 

 genus Estheria (fig. 202, G), which appears to be related to 

 the living Limnadia, and which in some respects constitutes 

 a connecting-link between the Phyllopods and the Ostracodes. 

 The body in Estheria is enclosed in a bivalve 'carapace (fig. 

 205, A), and the feet are foliaceous. The valves of the 



Fig. 205. A, Carapace of Estheria ovata, magnified six diameters Trias ; B, Carapace of 

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



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

 to one another along a dorsal line. From these circum- 

 stances, and from their being marked with numerous con- 

 centric lines of growth, the carapace-valves of Estheria very 

 closely resemble the shells of certain Bivalve Molluscs, 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 Estherice (fig. 202, G) 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 presumption that the de- 

 posits in which fossil Estherice occur were deposited in fresh 

 or brackish water ; but they not uncommonly occur in con- 

 junction with undoubted marine remains. They appear, on 

 the whole, to occur most frequently in those accumulations 

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

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

 Estherice occur in the Devonian, Carboniferous, Permian, 



