Bibliographical Notices. 45 
sometimes the union is more extensive. In a few genera, as Anatine 
and Pholadomya, the two lamelle of the same side are so united as to 
appear like a single gill. In the Pholadomya it forms a thick oblong 
mass, finely plicated transversely, attenuated at both extremities, 
slightly bifid at the posterior one. A line traverses longitudinally the 
middle of the external surface, which has no other trace of division. 
The branchiz on each side adhere to the mantle by the whole of their 
dorsal margin, and are united together where they extend beyond the 
visceral mass, being separated, by the interposition of that mass, 
along their anterior two-thirds. A narrow groove extends along the 
free anterior margins of each gill. When the inner side of this appa- 
rently simple gillis examined, it is Seen to be divided into three longi- 
tudinal channels, by two ridges, containing the vascular trunks and 
nerves of the gills. -.A style passed from the éxcretory siphon; behind 
the conjoined extremities of the branchig, enters,the dorsal channel, 
from which the excretory respiratory currents are discharged: the 
middie channel is characterized by an orifice which conducts into the 
cavity of the gill, where the ova are hatched: the third channel forms 
the inner or mesial surface of the gill, which is not otherwise divided.” 
—Ep. Ann. Nai. Hist.} 
BIBLIOGRAPHICAL NOTICES. 
The Genera of Birds. By G. R. Gray, F.L.S., illustrated by D. W. 
Mitchell, B.A., F.L.S. Imperial 4to. | Parts 1—14. 
Ir is now nearly twenty years since any naturalist has attempted to 
give a complete synopsis of the species of birds. The accessions to 
our knowledge during the interval have been very great; and the 
progress which has been made in elucidating the characters and im- 
proving the classification of the species previously known, is no less 
remarkable. The facts thus elicited were however scattered through 
rare and costly publications, many of which, especially the volumi- 
nous Transactions of foreign Societies, are almost inaccessible to the 
working naturalist, who will therefore hail with gratitude the work 
before us, which supplies him with a ready index to the whole sub- 
ject of ornithology. Mr. Gray’s position in the British Museum has 
given him peculiar facilities for perfecting his laborious undertaking, 
which requires a constant and ready access to books and specimens. 
The classification which he has adopted is for the most part con- 
sistent with natural affinities, though-some of the groups, Pachyce- 
phaline for instance, consist of rather incongruous materials. Really 
natural groups are generally confined within certain geographical 
limits, and when we see an assemblage either of species or of genera 
from remote parts of the world brought together to form a superior 
group, there is often reason to suspect that their supposed affinities 
are apparent rather than real. 
The definition of families and genera is one of the most difficult 
duties of the naturalist, and he is often unable so to generalize the 
characters of groups as to satisfy the logician. We frequently see a 
species connected by the closest affinity to others, yet differing from 
them in the verypoints in which the latter mutually agree, so that 
