176 D. J. SCOURFIELD ON THE WINTER EGG OF A RARE WATER-FLEA. 



which, as ah^eady stated, is perhaps the most highly evolved of 

 the proto-ephippia hitherto recorded. First, it is formed, as in the 

 Daphnidse, from a much more limited portion of the shell, bounded 

 approximately by a semicircle described upon the dorsal margin ; 

 secondly, it possesses a thick inner coat of specially formed spongy 

 tissue ; and thirdly, it is provided wdth large hook-like appendages. 

 In spite of these advances in complexity, however, it does not 

 quite reach the level of the Daphnidan ephippium, because its 

 outer coat is not specially altered beyond the mere deposit of 

 pigment and some extra chitin perhaps, whereas in typical 

 ephippia there is always a more or less abundant development of 

 closely-set hexagonal prismatic cells which are quite independent 

 of the original shell sculpture, and which, becoming readily filled 

 with ail', render the ephippium lighter than water. Nevertheless 

 it must be confessed that the proto-ephippium just described 

 goes far to bridge over the gulf which, until recently, seemed 

 to separate the protective coverings of the resting eggs of the 

 Daphnidfe from those of the Lynceidse, etc. 



What, it may now be asked, is the value of this very peculiar 

 type of proto-ephippium from the point of view of the systematic 

 subdivision of the troublesome family Lynceidse ? Considering 

 the extremely close relationship of the three known species of 

 Leydigia {L. acanthocercoides Fischer, L. quadi'angularis Leydig, 

 and L. australis Sars), it might have been expected that it would 

 at least have given us another good generic character. Un- 

 fortunately this does not appear to be the case, for Professor Sars 

 has placed on record that " no perceptible modification of the 

 dorsal part of the carapace was ever observed " in females of 

 L, aitstralis carrying winter eggs (6, p. 43). Unless this observa- 

 tion rests upon a wrong interpretation of the nature of the eggs 

 — which, however, scarcely seems possible, as Professor Sars dis- 

 tinctly says that they w^ere easily distinguished from summer eggs 

 by their dark-yellowish colour — the difference between L. acantho- 

 cercoides and L. australis in respect of the provision made for 

 their resting eggs is a most remarkable instance of the wide gulf 

 which may really separate two apparently closely allied species. 



In the title of this paper I have called L. acanthocercoides a 

 rare Water-flea, and that, I think, is quite allow^able. It has 

 been found so far by very few workers, and in nearly all cases the 

 records are accompanied by statements that only a few specimens 



