44 Rupert Jones,, on Bivalved Entomostraca. 



moveable projection above the eyes. In tbe Cladocera 

 {Daphnia, &c.) the carapace is still' more flatly folded down, 

 with a bend along the dorsal line ; and the whole of the body 

 is included within it, except that the antenna? (as swimming 

 limbs) protrude at the head from lateral notches, M'hich give 

 to the front of the carapace a hood-like or quaintly beaked 

 shape. 



In other Bivalved Entomostraca the two sides of the 

 folded carapace are quite distinct, forming separate valves, 

 but united in life along their dorsal margins by either a 

 simple membranous attachment (as in Estheria, Sec), or by 

 a more complex system of ridge and furrow, or teeth and 

 sockets (as in the Cyproidea). 



In outline the carapaces of Cladocera range from orbicular 

 to oblong, with varying contours. They are horny or chitinous, 

 thin, usually transparent, and ornamented often with some 

 reticulate pattern, having reference ' to the hexagonal cell- 

 system of the typical crustacean test, or the network resolves 

 itself into delicate bands and furrows by the greater develop- 

 ment of one set of mesh-lines than another. This carapace is 

 periodically moulted and renewed ; but occasionally it is re- 

 tained, and one layer succeeds on the inside and at the outer 

 edge of another until the valve is marked with several con- 

 centric boundary-lines of the periodic stages of growth. Mr. 

 Norman points out that this feature, normal in Menosphilus 

 tenuirostris, is occasional in Lynceus elongatus ; see ' Nat. 

 Hist. Trans. Northumberland and Durham,' 186T, p. 53. It 

 is also normal in the Limnadlada, which retain their valves, 

 v.'hilst they cast only a chitinous skeleton or framework of 

 the body. 



Fossil carapaces of Cladocera have not been recognised, 

 their extreme tenuity j^robably being neither favorable for 

 their preservation nor, if j^reserved, to their detection in the 

 fossil state. 



The Bivalved Phyllopods, such as Limnadia, Estheria, and 

 Lhnnetis, are larger than the Cladocera, and their valves are 

 usually thicker ai:d stronger. In shape round, oval, or 

 oblong, they often resemble the shells of Conchifera or 

 Bivalved ISIoUuscs, and have been mistaken for them when 

 living, and much more frequently in the fossil condition. 

 The presence of a straight hinge-line, of umbones, and of 

 concentric lines of growth, are special features in which they 

 more or less imitate the Conchifera, such as Avicula, TeU'wa, 

 Pisidium, &c. Estheria donaciformis came to the British 

 Museum as a Nucula ; but Dr. Baird recognised its crustacean 

 characters, disguised as they are by the molluscan shape. 



