514 APPENDIX. 
with the addition of a mass of the most minute anatomical 
and functional details—to. us so incomprehensible, that we 
have given up the attempt to unravel them. 
The key-stone of the theories of these gentlemen is the 
assumed permeability of the acephalan gill-lamine. I believe 
that this determination will prove erroneous. I am, of course, 
open to conviction, but I think my position founded on the 
rock of “ magna est veritas, et preevalebit.” 
Under these circumstances I cannot, in the present phase 
of this question, withdraw a single position of the branchial 
theory I have proposed. 
Anatomy, even the microscope, chemical tests, and the 
most ingenious experiments, have hitherto failed demon- 
stratively to solve this long-vexed question ; all our attempts 
amount to little more than hypothesis, intrepid assertion and 
assumption, which often simulate facts, and pass for proofs. 
DeENTALIUM TARENTINUM.—(P. 235.) 
I have stated that “M.Deshayes has omitted to mark the 
vein which runs at the dichotomous poimts of his organ, 
which, when viewed under high powers, is very visible, and 
which I take to be the branchial vein.” 
I withdraw this observation, whether my conjecture be 
correct or not, as it would have been inconsistent in that 
naturalist to figure a branchial vem in an organ considered 
by him to be the liver. 
His liver is our branchie. We have lately re-examined the 
Dentalium Tarentinum, and we almost think that the artery 
and vein have a nearly parallel position on the dorsal line, as _ 
in the Bivalves. The tumid cord-like appearance of the lower 
border of this minute organ has perhaps led to an erroneous 
conclusion. 
CHITON CANCELLATUS.—(P. 251.) 
Cuiton aLBus.—(P. 251.) 
Though the above are admitted as distinct species, we are 
now bound to add, that this determination must be considered 
provisional ; there is reason to believe that both these forms 
