10 NATURAL SCIENCE [July 
when the red hypodermis is examined by the spectrum eye-piece by 
means of reflected light. 
We fail to find any evidence in this paper that the ‘ chemistry 
of the crustacean pigments has been advanced beyond the old stage 
of solution and colour-change with reagents. What we want is 
isolation and purification before we can say that our knowledge of 
the subject has been largely increased. 
However, in spite of these minor blemishes (and all new work 
must be imperfect), we welcome this paper and hope the author may 
continue her observations. 
THE EMBRYONIC SHELL IN BIVALVES 
THE shell, or rather pair of shelly valves of the young bivalve, that 
forms before the little creature is hatched, is usually very different 
in appearance to that enclosing the fully formed animal. The 
prodissoconch, as it has been termed, is small, unornamented, and 
may frequently be seen capping the umbones of the older shell. 
It has been considered the sole stage prior to the exclusion of the 
embryo from the egg-shell. Mr Félix Bernard, however, (Comptes 
Rendus Acad, Sci. Paris, vol. exxiv. pp. 1165-8) has detected an earlier 
stage which he calls the protostracum, and he has been able to find 
this protostracum on the summit of each prodissoconch studied by 
him. The Glochidium stage in the Unionidae is the equivalent of 
this protostracum. In most cases the hinge-line of the protostracum 
undergoes no increase during the completion of the prodissoconch 
stage. It consists of a series of interlocking crenulations, for which 
Mr Bernard accepts Dall’s suggested name of provinculum, with a 
central ligament pit. The development of the Heterodonts is so rapid, 
that, as a rule, the crenulations have no time to form, but are 
superseded by the true teeth, whose development has already been 
commented on in these pages (vol. ix. p. 358), This acceleration of 
development of the hinge is correlative with the earlier incubation 
of the individuals of this group, entailing an earlier use of the shell 
with a consequent stimulation of the hinge through function. 
The free swimming larval stage, which has its foot adapted for 
creeping, and free mantel lobes, guiltless of siphons, and whose gills 
are situated in the rear, whilst it possesses the velum characteristic 
of all larval mollusca, is common to all Pelecypoda and representa- 
tive of the ancestral form. 
THE REPRODUCTION OF DIATOMS 
THE two chief methods by which diatoms have hitherto been 
supposed to reproduce. themselves are (1) by simple vegetative 
