82 Prof. K. Mobius on the 



The shell-bearing Mollusca, to which the second volume of 

 the ' Fauna der Kieler Bucht ' is devoted, are particularly well 

 fitted for comparisons between animals of the same species 

 inhabiting both the North Sea and the Baltic. In all the 

 shells are lighter than in specimens of the same dimensions 

 from the North Sea*. Stunting occurs also in the other 

 classes of animals. Thus at Kiel the fore part of the body of 

 Temora longicornis is, on the average, only 1 millim. in 

 length, whilst it becomes 2 millims. long in individuals from 

 Arendal. At Kiel Pectinaria belgica only attains a thickness 

 of 5 millims. in front ; at Arendal this worm becomes so 

 large that it attains a transverse diameter of 12 millims at its 

 anterior end. Another worm, Travisia Forbesii, becomes 15 

 millims. long, and 3-4 millims. thick at Warnemunde, and 

 26 millims. long and 7 millims. thick (according to B-athke) 

 on the Norwegian coast. 



In the eastern basin of the Baltic the animals become far 

 more stunted than in the western. Near Kiel Mytilus edulis 

 becomes 8-9 centims. long ; in the eastern basin {e.g. on the 

 Stolper Bank, near Gotland, near Dularo) this mollusk only 

 attains a length of 3-4 centims. My a arenaria, Tellina bal- 

 thica, and Gardium edide differ less in the eastern basin, as 

 far as Gotland, from the individuals of the same species in the 

 western basin, than the individuals of Mytilus in the two 

 basins differ from each other. This phenomenon is due to the 

 fact that even in the western basin these mollusca are sur- 

 rounded for the greater part of the year with but slightly salt 

 water, as they inhabit the smaller depths. 



In Mytilus edulis and Tellina balthica, of the eastern basin, 

 the calcareous layers of the shell are extraordinarily thin. 

 After the death of the mollusk, the calcareous mass of the 

 shell seems to disappear very speedily ; for among the rocks of 

 eastern Sweden, between Sweden and Gotland, and in the Cal- 

 marsund, we found in the clayey mud of the sea-bottom a 

 great many cuticular membranes of Mytilus edulis and Tellina 

 balthica most perfectly preserved. The two brown membranes 

 were often united at the dorsal margin by the ligament, as in 

 the perfect shell. If a sea-bottom of this kind were upheaved 

 these cuticular shells in the clay would appear just like thin 

 impressions of Posidoniw, with all the curvatures and deforma- 

 tions by pressure with which we are acquainted in those fossils 

 of the fine shales of secondary formations ; and we should fall 

 into a great error if we were to conclude from these bendings 

 of the cuticular shells that the strata of clay had been bent 

 after they were laid dry. 



* See ' Fauna der Kieler Bucht,' Bd. ii. p. xvii. 



