10 NATURAL SCIENCE [July 



when the red hypoderniis 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 Felix Bernard, however, (Comptes 

 Renclus Acad. Sci. Paris, vol. cxxiv. 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 superseded by the true teeth, whose development has already been 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 Eeproduction of Diatoms 



The two chief methods by which diatoms have hitherto been 

 supposed to reproduce themselves are (1) by simple vegetative 



