316 THE AMERICAN MONTHLY [Dec 
ordinary shell markings on the valve, but, from the re- 
lation of the latter to oneanother on each side of the line 
it is evident that a considerable amount of alteration of 
the original shell sculpture has taken place. The prob- 
ability is that a line of hexagons has beep suppressed or 
rather completely modified to form the line of weakness, 
and this view is borne out by the arrangement of the shell 
markings shown in fig. 7, where we seem to have a very 
early stage of the process exhibited. The one long cell, 
which lies exactly where a portion of the line of weakness 
is always developed, has plainly been formed from’ the 
ordinary hexagonal markings, because its edges show the 
characteristic zigzag arrangement, and the crossbars are — 
also just discernible, although almost obliterated. ‘It is 
somewhat strange that in the specimen from which this 
was drawn there was no trace of any modification having 
commenced for the production of the lateral bands and 
slits. It looks as if the production of a line of weakness 
may be older in the history of the development of these 
ephippia than the formation of the lateral chitinous bands. 
This may very well be so, because among the Lynceida, 
where there is in many cases scarcely any actual modifi- 
cation of the shell, there is almost invariably a line of 
weakness developed prior to the moulting of the ephip- 
plum. 
Having now seen how it is that the ventral portions of 
the valves become so easily detached from the rest of the 
shell when an ephippium is formed, we may very well 
ask ourselves why this process should take place. ‘As 
the phenomenon is so common among the Cladocera there 
must be some fundamental necessity for it. I think the 
answer to this question is undoubtedly to be found in the 
fact that in the vast majority of cases it would be quite 
impossible for the anterior margins of the valves to be 
brought into contact if the ventral, and especially the 
anterior ventral portions of the valves, owing to their 
