82 Mr. T. R. Jones on some species of Leperditia. 



Later researches, however, have greatly extended the list of 

 the Entomostraca ; and there are several well-known existing 

 forms which possess a bivalved carapace or shell ; such as Cypris, 

 Cythere, Cypridina, Limnadia, Daphnia, and Nebalia. In these 

 the carapace is vertical, and its two lateral halves or valves are 

 either distinct in themselves and united by a more or less elabo- 

 rate dorsal hingement, — or form one entire shell, doubled 

 along the back with a flexible fold. Other Entomostraca have 

 the carapace horizontal and flattened, or nearly so, and marked 

 by a mesial ridge or line along its greater axis ; — as Apus and 

 Limulus. Of the above-mentioned genera, nearly all have been 

 presented in geologic times in forms more or less closely related 

 to the existing species. 



In the case of the fossil Entomostraca, the soft parts, inclu- 

 ding the maxillary, branchial, and locomotive organs, on which 

 the generic and sometimes the specific distinctions of the recent 

 forms are mainly established, have quite disappeared ; and the 

 hard carapace-valves alone remain to guide us in the recognition 

 of genera and species. It is fortunate, however, that the families, 

 and most of the genera even, of the existing Bivalved Entomos- 

 tracans * have carapaces sufficiently characteristic to enable us 

 to co-ordinate the fossil forms by the analogies presented in the 

 form and structure of the valves. 



For the most part, the Entomostracan bivalves, both from 

 their minute size and in their general aspect, are strikingly dif- 

 ferent from the Conchifera. Some, however, as for instance the 

 Estheria donaciformisf, may readily be mistaken for ordinary 

 bivalve shells. 



One of the two little Gothland fossils above mentioned, 

 although it resembles the Conchifera in being bivalved and in 

 its bean-shaped form, diff*ers from Molluscan bivalves { in 

 general appearance, and in the combination of the following 

 characters ; — the great length of the hinge-margin, — the absence 

 of umbos, — the extreme overlapping of one valve over the other 

 on the ventral margin, — the uniform smoothness of the surface, 



* When we refer to minute distinctions of form, hingement, and orna- 

 m£ntation, we find that among the recent Bivalved Entomostraca some 

 famiUes and even genera have carapaces pecuhar to them {Cypridina, Ne- 

 balia, Limnadia)', whilst in other families a nearly similar carapace belongs 

 to two genera {Cypris and Candona, — Daphnia and Lynceus); and on the 

 contrary even two characteristically different carapaces occur among the 

 species of one genus {Cy there and its subgenus Bairdia). 



t Proceed. Zool. Soc. 1849, vol. ii. p. 86. PI. Annulos. 11. 



X With respect to the characters in which there is an approach in form, 

 viz. a straight hinge-line and an overlapping of one valve over the other, I 

 have been favoured with the following note by my friend Mr. Pickering : — 



" On looking over the recent Conchifera, or bivalve MoUusca, I do not 



