Notes on the Fossil Ostracoda. 168 



of water being driven over it by the movement of the fan- shaped 

 organ spoken of above. The genital organs of the male are 

 of a very complicated structure. Cypris is remarkably prolific, 

 a single congress with the male being apparently sufficient in 

 Bome species to render the females fertile for life. There is also 

 strong reason to believe that parthenogenesis exists for several 

 generations in some members of the group. 



The majority of the Ostracoda produce eggs, some carrying 

 them about within the shell, others (the minority, Cypris, &c.), 

 fixing them to water-plants. Glaus, to whom we are indebted 

 for much recent knowledge of the structure of Cypris, tells us 

 that the young of this genus pass through a most comphcated 

 metamorphosis. In the first or "nauplius" stage [name given 

 by F. 0. Miiller to unsegmented ovate larva of lower Crustacea, 

 with median frontal eye, but without a true carapace] Cypris 

 possesses only three pairs of hmbs and a bivalved shell. It 

 passes through several later stages, with complete moults, before 

 arriving at maturity. The Cythenda (marine) are much more 

 fully developed at birth. 



Unfortunately, in dealing with fossil Ostracoda, we are, except 

 in the rarest cases, quite unable to learn anything about the 

 animal itself, and when studying the numerous and peculiar 

 extinct genera of palaeozoic times, we are forced, if we attempt 

 to classify and arrange them, to content ourselves with the 

 external evidence afforded by the shell. This is often very 

 puzzling and unsatisfactory, as certain structures and peculiarities 

 of the shells are difficult to understand by analogy. It is also 

 extremely difficult in a few cases to definitely decide upon the 

 anterior and posterior ends of the shell in some of the extinct 

 genera. Professor Eupert Jones has found it a fairly safe guide 

 to take the narrow and compressed end as the anterior, in that 

 it ofi"ers the least resistance to the medium in which the animal 

 lived. But in many of the silurian Beyrichim the front end is 

 swollen by the addition of a great lobe, on each side, to twice 

 the size of the posterior, and therefore we have at once an 

 exception to a possible rule. 



The difficulty in the way of our obtaining knowledge of the 

 softer parts of the fossil Ostracoda will make it necessary for me 

 to describe in some detail the structure and peculiarities of the 

 carapace. The carapace of the Ostracoda is composed of two 

 valves, one usually larger than the other, which are joined 

 dorsally and kept apart ventrally by an elastic ligament. The 

 closing of the shell is effected by an adductor muscle, so that in 

 this respect the Ostracoda resemble the bivalved Mollusca. 

 The adductor muscle is placed towards the anterior end of the 

 carapace, and is inserted in a series of sunken. pits in the median 

 line of each valve. These spots, "muscle-pits" or "lucid-spots," 



c 



