43 Mr. J. D. Dana on the Origin of the 



curved sharply (fig. 9 a). In Buccinum, as just stated, the 

 dorsal border terminates in a straight point. Although this 

 trait is only a microscopic incident of formation, it is quite 

 enough to constitute the distinctness of the species. In every 

 other respect the laminae discover the same structure as that just 

 explained in Buccinum. The gill-leaves of Murex and I^assa 

 exhibit also slight variations of shape, compared with the 

 standard figure of those of Buccinum. From such examples the 

 naturalist may well exclaim, how marvellous and unaccountable, 

 that in estabUshing the independence of species, Nature should 

 change the very fabric of the minutest parts of the body ! 



In the Conidse and Cyprseadse, the author has every reason 

 to believe that the branchiae conform with exactness to the type 

 of those of Buccinum. They may vary in the special outline of 

 the laminae, but not in essential structure. They lie in the 

 branchial chamber in the same oblique position, and exhibit 

 the same relation to the glands of the cavity. 



Although the transition may be strange and abrupt, it is con- 

 venient at this point to pass to the consideration of the pulmo- 

 niferous Gasteropods ; not because there is much in common 

 between their breathing system and that of the branchiferous 

 orders, but because the glands contained in the thoracic cavity 

 of the Pulmonata correspond in structure and dependencies most 

 intimately with those which are contained in the branchial 

 chamber of the Pectinibranchs last described. According to 

 this distribution of subject, the '^ glands ^^ of the respiratory 

 cavity of both the branchiferous and pulmoniferous orders will 

 come to be considered under one head. 



[To be continued.] 



V. — On the Origin of the Geographical Distribution of Crustacea. 

 By Mr. James D. Dana. 



[The present paper is the conclusion of Mr. Dana's Report on the 

 Geographical Distribution of the Crustacea. As the opinions here 

 put forward appear to be of great importance in the study of the 

 geographical distribution of animals, we have thought it advisable to 

 transfer this portion of Mr. Dana's work to our pages; but the tables 

 of facts on which they are founded would occupy too much space ; 

 they will be found in Silliman's Journal, vols. xvi. xviii. xix. & xx. 

 —Eds.] 



The origin of the existing distribution of species in this depart- 

 ment of zoology deserves attentive consideration. Two great 

 causes are admitted by all, and the important question is, how 

 far the influence of each has extended. The first is, oiiginal 

 local creations ; the second, migration. 



Under the first head, we may refer much that we have already 



