238 D. J. SCOURFIELD ON EPHIPPIA OF LYNCEID ENTOMOSTRACA. 



In the great majority of cases the ephippinm is about three- 

 fourths of the area of the old shell, and the portion detached 

 does not include the whole of the ventral margin. In this the 

 ephippia of the Lynceidae agree with those of the Bosminidae 

 (see 11), but are in sharp contrast to the ephippia of the 

 Daphnidae, in which no part of the ventral or even posterior 

 shell margins is ever used in the production of the ephippium. 

 In a few cases of Lynceid ephippia {e.g., Alo7iopsis ambigua, 

 Alona rectangida) only from two-thirds to a half of the shell is 

 utilised, the detached portion including the whole of the ventral 

 and a part of the posterior margin, and in one case [Leydigia 

 acanthocercoides) the ephippium is actually cut out of the old 

 shell without including any portion of the ventral or posterior 

 margins (except in the form of special hooks) just as in the 

 Daphnidae. On the other hand, there is one species at least 

 {Alona tenuicdudis) which utilises the whole of its shell as an 

 ephippium. 



(2) The dorsal margin of th^e ephippium is always specially 

 strengthened by a more or less copious deposit of chitin. 



In most cases the chitin appears, when viewed from the side, 

 as a clear, highly refractive band; but in a few species {e.g., 

 Alonella rostt^ata. A. excisa) the extra thickening is not very 

 marked, although it no doubt exists. This feature of the 

 thickening of the chitin along the back is found also in ephippia 

 other than those of the Lynceidae, and may be quite general. 

 In the Bosminidae, however, it appears to be confined to a definite 

 narrow band, while in the Lynceidae, and apparently also in the 

 Daphnidae and Lyncodaphnidae, there seems to be a general 

 thickening of the chitin over the dorsal part of the shell. 



(3) The anterior margin is formed by the ordinary line of 

 junction between the head-shield and the valves. 



This is probably true in the case of all ephippia, and in some 

 instances it gives quite a characteristic appearance to the 

 ephippium — e.g., Camptocercus rectirostris, Dunhevedia, and species 

 of Pleuroxus and Chydorus. 



(4) The line of separation between the ventral portions of the 

 valves to be detached and the ephippium is traced out, at least 

 anteriorly, before the ephippium is thrown off, and it appears to 

 be always due to the special formation of a row or rows of easily 

 separated pieces of chitin, of variable but usually minute size. 



