Prof. W. King on the Fossil genus Anthracosia. 51 



same extended influence as in that of Crustacea. Mollusks, if 

 we except oceanic species, are no travellers, and keep mostly to 

 narrow limits. 



XIII. There is evidence, in the exceedingly small number of 

 torrid-zone species identical in the Atlantic and Indian Oceans, 

 that there has been no water- communication across from one to 

 the other in the torrid zone, during the period since existing 

 species of Crustacea were first on the globe. 



XIV. As to zoological centres of diffusion for groups of spe- 

 cies, we can point out none. Each species of Crustacea may 

 have had its place of origin and single centre of diffusion in 

 many and perhaps the majority of cases. But we have no reason 

 to say that certain regions were without life, and were peopled 

 by migration from specific centres specially selected for this 

 end. If such centres had an existence, there is at present no 

 means by which they may be ascertained. The particular tem- 

 perature region in which a species originated may be ascertained 

 by observing which is most favourable to its development : we 

 should thus conclude that the Ranina dentata, for example, was 

 created in the subtorrid region, and not the torrid, as it attains its 

 largest size in the latter. By pursuing this course with reference 

 to each species, we may find some that are especially fitted for 

 almost every different locality. Hence we might show, as far 

 as reason and observation can do it, that all regions have had 

 their own special creations. 



The world, throughout all its epochs in past history, has been 

 furnished with life in accordance with the times and seasons, 

 each species being adapted to its age, its place, and its fellow 

 species of life. 



VI. — On Anthracosia, a Fossil Genus of the Family Unionidse. 

 By William King, Professor of Mineralogy and Geology 

 in Queen's College, Galway, Corresponding Member of the 

 Natural History and Medical Society of Dresden, &c. 



[With a Plate.] 



The lakes, rivers and estuaries of the Carboniferous period were 

 inhabited by two groups of Bivalves ; the generic characters of 

 neither have as yet been fully described. One group includes 

 forms having much of the external aspect of ordinary species of 

 Unio; while the other comprises members possessing the out- 

 ward appearance of certain aviculoid forms of Modiola, 



But as external resemblances are not always to be depended 

 on in determining the genus of a fossil shell, some palaeonto- 

 logists have gone no further, in the present case, than merely 

 to refer the bivalves in question to the genera named; while 



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