454 Sir H. H. Howorth — Geological History of the Baltic. 



but not now living in this region, 14 of whicli are Arctic, 9 Boreal, 

 and 22 Lusitaiiic. 



The 14 Arctic forms are : — 



Terebratella Spitzbergensis, Pectenislandiciis, Portlandia lenticula (?) , Tridonta 

 borealis, Panopcea Norvegica, Molleria costulata, Margarita Groenlandia, 

 M. cineraria, Morvillia undata{?), Marsenina microviphala, Trichotropis 

 borealis, Littorina palliata C?) , Sipho togatus (?) , and JJtriusculus per tenuis. 



The 9 Boreal species are : — 



Gwynnia capsula{?), Montacuta Voringii, Solen ensis{?), Cadulus propinquus, 

 Lamellaria latens, Auriculina diaphana, SipJio gracilis (?), Neptunea 

 antiquai^), and Utriculus obtusus. 



The 22 Ltjsitanic species are : — 



Area tetragona, Cardium tuberculatum. Tapes decussatus, Lepton squamosum, 

 Scrobicularia piperata, Lascea rubra, Tellina crassa, Macoma fabula, Psam- 

 mobia vespertina, Solecurtus antiquitatus, Pholas Candida, Cingula soluta, 

 Onoba vitrea, Alvania reticiilata, Aclis ascarif, A. unica, Turbinella lactea, 

 Odostomia albella, Cidinella nitidissima, Clathurella purpurea, Mangelia 

 nebula, Philine pruinosa. 



jer, op. cit., pp. 577, 578.) 



Some species like Isocardia cor, existing both in the beaches and 

 living, were formerly very common in the Christiania Fjord, but are 

 now very rare; others are decidedly rarer now than they were, such 

 as Pecte7i varius, P. septemradiatus, P. opercularis, and the oyster. 



In one matter I would take exception to Brogger's classification of 

 the later beaches, which he groups together under the name post- 

 Glacial beds. Having named the greater number of them Tapes beds 

 from the presence in them of that very characteristic shell, he 

 proceeds to treat others, which agree with the Tapes beds virtually 

 in all their other contents but in which the Tapes have not been 

 found, as belonging to a different horizon. This seems to me to 

 introduce a quite unnecessary complication into the problem, 

 unjustified by the evidence. There are only a limited number of 

 molluscs which are sufficiently elastic to live under very different 

 conditions of food and bottom, and the occasional absence. of a 

 particular shell is often merely due to peculiar local conditions. 



Every shell-collector knows as an elementary fact that when we 

 pass from clay to mud or sand or gravel or rocks, we at once lose 

 certain of the species which are perfectly contemporaneous and 

 which are now living in different parts of the coast of the same 

 sea. The complete absence of the Tapes and the oyster from the 

 modern Cattegat, although they were abundant there before the 

 Baltic breach, shows that they were not capable of tolerating certain 

 changed conditions like others of their companions could, and we 

 must in such cases take the general facies of the contents of the beds, 

 and not the presence or absence of a particular shell, as justifying us 

 in creating a new horizon. 



In regard to the dying out of the forms it is not easy to give an 

 entirely satisfactory explanation, for they belong to all three of the 

 geographical provinces into which the mollusca have been divided 

 by Brogger. It is possible that the reduction in the salinity of the 



