I $8 ANNULOSA. 



ESTHERIA. Before going on to the order Phyllopoda, we 

 may briefly notice here the little fossils belonging to the genus 

 Estheria, as these are in some respects closely allied to the 

 Ostracoda, or are intermediate between these and the true 

 Phyllopods. Upon the whole, however, Estheria would seem 

 to be most nearly allied to the living Limnadia, in which case 

 it would be rightly referred to the Phyllopoda. The body in 

 Estheria is enclosed in a bivalve carapace (fig. 101, A), and 



Fig. 101. A, carapace of Estheria ovata, magnified six diameters, Trias ; B, cara- 

 pace of Leaia Leidyi, magnified five diameters, Lower Carboniferous (after Rupert 

 Jones). 



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, for which they have often been mis- 

 taken. The valves are usually sub-triangulfr, ovate, or sub- 

 quadrate in form, and they possess a horny texture. 



The living Estherice are, without exception, inhabitants of 

 fresh or, rarely, brackish water j and no one of the recent 

 twenty-four species has been detected in the sea. This would 

 afford a strong presumption that the deposits in which fossil 

 Estheria occur were deposited in fresh or brackish water ; but 

 they not uncommonly occur in conjunction 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 inundations, and of more perman- 

 ent 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 genus Leaia (fig. 101) is very nearly allied to Estheria, 



