144 
The Application of 
distinct branches. The Prosobranchiata and perhaps the Hetero- 
pods being the descendants of one ; the Opisthobranchs and Pul- 
monates of another ; and the Chitons of a third. From one of 
these, or perhaps from the branch now represented by DentaHum, 
the Lamelhbranchs seem to have been derived at a very early 
period, and to have diverged considerably from the ancestral form, 
becoming degraded in certain respects and at the same time special- 
ized in others. 
In this scheme all reference to the Tunicata is omitted, since it 
will be conceded by all embryologists that, whatever the affinities 
of this group may be, they are certainly not moUusks. 
I have already referred to one serious objection to the view here 
advocated ; that is, that it fails to account for the remarkable em- 
bryonic forms of certain Pteropods. Huxley has advocated the 
view that the Pteropoda and Dentalium have an annelidian an- 
cestry distinct from that of the remaining Mollusca. This view 
would help us to understand the remarkable larval form of such 
genera as Pneumodermon, and at first sight would seem to present 
a way of escape from our difficulty. It fails to account for the 
perfect agreement between the veligers of the thecosomatous Ptero- 
pods and the Gasteropods, however, and thus introduces a difficulty 
at least as great as that which it removes. At present the safest 
plan seems to be that of waiting' for more knowledge, bearing in 
mind the existence of this at present insoluble difficulty. — From 
the Proceedings of the Boston Natural History Society^ 
Feb. 1876. 
V. — The Ap'plication of Photography to Micrometry, with special^ 
reference to the Micrometry of Blood in Criminal Gases. 
By J. J. Woodward, M.D. (U.S.A.). 
Eeoent experiments in photographing the blood-corpuscles of man 
and other animals lead me to propose photography as affording the 
means of making comparative measurements more accurately, and 
with less expenditure of time, than can be done by any other 
method. 
The plan I propose is simj)ly as follows. The blood is placed 
on a glass stage micrometer and photographed with any convenient 
power, both blood and micrometer appearing sharply defined in 
the picture. The measurements are then made on the negative. 
The stage-micrometer must be ruled on the upper surface of a 
piece of glass, and must have no thin cover. For mere compara- 
tive measurements any ordinary stage-micrometer in which the 
lines are equidistant can be made to answer by simply removing 
