26 AMERICAN JOURNAL 



OBSERVATIONS ON THE BATHYMETRICAL AND GEO- 

 GRAPHICAL DISTRIBUTION OP MARINE INVERTE- 

 BRATE ANIMALS. 



BY J. GWYN JEFFREYS. 



(From "Eeport on Dredging among the Hebrides," published in the 

 Annals and Magazine of Natural History, Nov., 1865.) 



Some of our most conspicuous and prized shells, that are also 

 of a northern type, are wanting in the Hebrides. Saxicava 

 Norvegica, Natica Groenlandica, Biiecinum Humphreysianum, 

 Bn^cinopsis Dalei, Fusus Norvegicus, F. Turtoni, and F. Ber- 

 nieiensis are in this category. All the above (with the excep- 

 tion of Buccinum Humphrey sianuvi, which inhabits Shetland 

 and the coasts of county Cork) are met with on the Dogger bank ; 

 and the first two are fossil in the Clyde beds. Six out of the 

 seven being univalves, I would venture to surmise that their non- 

 existence in the western seas of Scotland may have arisen from 

 the circumstance that the diffusion of univalves is slower than 

 that of bivalves. The spawn of the former is attached to the 

 spot where it is shed, or in a few cases (e. g. Capuhis and Calyp- 

 trcea) it is hatched within the shell of its sedentary parent ; so 

 that the fry forms a colony, and need not roam to any distance, 

 provided their station yields a sufficient supply of food and has 

 the other requisites of liabitability. Not so with bivalves. These 

 shed their ova into the water, or else (as in some of the Kellia 

 family) hatch them within the folds of the mantle, whence they 

 are excluded on arriving at maturity. Their fry swim freely 

 and rapidly by means of numerous encircling cilia. The meta- 

 morphic state lasts many hours. During that period they can 

 voluntarily traverse considerable distances, or they may be invol- 

 untarily transported by tidal and oceanic currents. Time is the 

 only element necessary for their widest dispersion over the adja- 

 cent seas, if no barrier intervenes. Should, however, such an 

 obstacle present itself, whether in the shape of previously exist- 

 ing dry land, like that which separates the North Sea from the 

 Atlantic, or from an upheaval and drying-up of the neighboring 



