Dr. 0. A. L. Morch — On the Crag of Iceland. 395 



indicates a warmer climate, like that of the Baltic near the coast of 

 Mecklenburg, where a small variety is said to exist. The size of 

 the specimens of the Crag-formation of Iceland is in general not 

 remarkable, except the following species, which may be considered 

 dwarf varieties : Fusus Olavii (perhaps dwarf variety of F. gracilis), 

 Cyprina Islandica pumilio, Gardium Grcenlayidicum, Tritonium Groen- 

 landicum, Ch., Tritonium undatiim pumilio. 



The following species may be considered very large in the Crag- 

 formation of Iceland : Actceon Noce, Natica aperta, Pleurotoma turricula, 

 Gorhulomya, two or three species, much larger than any specimen 

 known to me from England. The three species of Mactra may be 

 considered large in comparison with the species now living in the 

 North Sea. 



The dwarfed condition of certain species may depend on various 

 causes. The North Sea shells all become smaller when they 

 enter the Cattegat and the Baltic, diminishing in size according to 

 the decrease of the saltness of the water. The freshwater shells, as 

 Neritina and Lymncea, decrease, on the contrary, in the same pro- 

 portion as the water becomes Salter. 



List of Mollusca found in tJie Crag -formation of Iceland at 

 Hallbjarnastadir. 

 (The species marked with * are not known to me from autopsy). 



1. ActoBon NocB, Sowb., Min. Conch., tab. 374. 



Tornatella Norn, Nyst, Coq. Foss. de Belgique, p. 424, tah. 37, fig. 22. 

 One specimen 16 mm. long, and 10 mm. in diameter ; and another 

 smaller : Suturis depressis, subcanaliculatis, labro intus sulcato. 



2. Acrybia lutea, Gould. 



Natica aperta, Loven. 

 Natica Steenstrupiana, Winkler, I.e. p. 209. 

 Bulhus apertus, Loven, Morch, apud PaykuU. 

 Tungabakker Tjorn^s. The largest specimen is 32 mm. long and 

 25 mm. in diameter. 



3. Natica (Lunatia) Groenlandica, Beck. 



Natica catena, D. Costa, Wood, Winkler, p. 208. 

 Mamma et Lunatia Grcenlandica, Morch, apud Payk. 

 Common (Winkler, Paykull). 

 '•*4. Natica (Lunatia) varians, Dnjardin. 



Natica varians, Wood, p. 142, tab. xvi., fig. 6. 

 Natica borealis. Gray ? 

 Two specimens ; 30 mm. long, 25 mm. in diameter ; aperture, 

 21 mm. long, 13 mm. in diameter. 

 ^'*5. Natica {Lunatia'?) hemiclausa, Sowb., tab. 479, fig. 2. 

 Natica hemiclausa, Winkler, I.e. p. 209. 

 Eare (Winkler). 

 ''•*6, Natica occlusa, Winkler. 

 Natica occlusa, Winkler. 

 Common (Winkler). 



7. Natica clausa, Sowb. 



Natica clausa, Sowb., Winkler, p. 209, w. 6, Morch, Payk. 

 Common (Winkler, Paykull.) 



8. Natica {Neverita ?) . 



One specimen, 29 mm. long, 25 mm. in diameter ; aperture, 20 mm. 

 Perhaps the N. hemiclausa of Winkler. 



