PHOLAS. 203 
appearance, but no fluid escaped from them into the branchial 
sac.” 
It is proper to state, that the Pholas crispata is the species 
that has furnished my controversialists with their remarks on 
my branchial theory, which is illustrated chiefly by the P. dac- 
tylus. I am not aware that this circumstance is of much 
moment, as we may safely conclude that the gills of all the 
Pholades have im essentials the same character. But I ought 
to mention, that the framework of the respiratory apparatus 
in some tribes of the Bivalves presents a very different ar- 
rangement. For example, there are several British families, 
whose species I have seen alive, and which fortunately can be 
obtained, that have a peculiar branchial construction, which 
appears as to general configuration closely analogous to that 
lately described in the ‘ Annals’ to exist in the Chamostrea 
albida and Myochamna anomioides of authors, but the particular 
parts of the mechanism in my species do not accord; I think 
the narrow reticulated ribands on the external surface are 
not permeable, and do not he on apertures that communi- 
cate with the imterbranchial tubes. I refrain, at present, 
from extending these remarks; but I shall be prepared with 
some comparative notes on certain species that have only a 
single complete gill-lamina and a rudimentary one on each 
side of the body, which seem to me to differ essentially in 
structure from the descriptions that have been promulgated 
on the composition of the branchial mechanism of the species 
that have been alluded to. 
I now enter on the counter-statement to the last quotation, 
and beg to observe, that Messrs. Alder and Hancock, in the 
explanatory sketch of their Pholas crispata, ‘ Annals,’ pl. 15, 
vol. vii. N.S., give a very intelligible outline of their theory. 
Though entirely dissenting from it, I cannot but admire the 
ingenious delineation, particularly fig. 3, of the gill-lamine, 
showing the aspect of the meshes; it has, however, one fault 
—it exhibits them a// with symmetrical longitudinal fissures 
called “ orifices,’ which I think are ruptures of the mem- 
brane of each mesh, not one of which exists naturally in the 
three species I have examined. 
