Bibliographical Notices. 45 



sometimes the union is more extensive. In a few genera, as Anatina 

 and Pholadomya, the tivo lamellte 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 branchiae 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 gill is 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 excretory siphon, behind 

 the conjoined extremities of the branchiae, enters the dorsal channel, 

 from which the excretory respiratory currents are discharged : the 

 middle 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 other u^ise divided." 

 — Ed. Ann. Nat. 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. 

 It 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 aflSnities, though some of the groups, Pachyce- 

 phalince 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 aflanities 

 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 diflfering from 

 them in the very points in which the latter mutually agree, so that 



