GEOGRA PHICA L D IS TKIR U TIOX. I 3 / 



Tw/n'o^i^/ossa (Zittcl's classification), contains twenty-six fami- 

 lies ; of these, four families contain strictly fresh-water species, 

 a few genera of which are amphibious. One of the fami- 

 lies is made up of land species; the remainder are marine 

 forms. Species of, at least, three families have been taken 

 from the ab\'smal zone. If we consider only the genera 

 characteristic of the several zones, we find them distributed 

 among different families. Three of these are represented in 

 the 1st, or abysmal zone; four in the 2d, or deep-sea Coral 

 zone; five in the 3d, or Nullipore zone; five in the 4th, or 

 Laminarian zone; five in the 5th, or littoral zone. 



Genera of the Ctenobranchina characteristic of the Several 

 Bathymetric Zones. — The genera which have already been 

 found to characterize the several zones have been tabulated 

 (from lists derived from various sources) by Fischer." 



It will be noticed that some genera are restricted to single 

 zones, and others characterize the faunas of more than one 

 bathymetric zone. Examination of these lists shows the fol- 

 lowing genera of Ctenobranchia to characterize the faunas of 

 the respective zones. 



(i) T/ic Littoral Zone. — From high water to a depth of 

 12 metres, species of the genera Littorina, Hydrobia, Assi- 

 minea, Rissoa, Truncatella, Cerithium, Natica, Pyramidella, 

 Nassa, Purpura, Murex, Conus. 



(2) The Laminarian Zone. — From low tide to 15 fathoms; 

 a zone characterized by species mostly phytophagous, of the 

 following genera, viz. : Phasianella, Xenophora, Triforis, 

 Rissoa, Aclis, Daphnella, Lacuna, Terebellum, Pterocera, 

 Marginella, Mitra, Nassa, Phos, Drillia, Pleurotoma. 



(3) The Nnllipore Zone. — From 15 to 20 fathoms; the 

 zone of calcareous alg^t. The characteristic species are 

 mainly carnivorous, and of the following genera, viz. : Bela, 

 Buccinum, Cassis, Cassidaria, Chenopus, Eulima, Fossarus, 

 Fusus, Nassa, Natica, Pleurotoma, Trichotropis, Tritonium, 

 Trophon, Velutina. 



(4) The Braehiopod Zone, or that of deep-sea corals, ex- 

 tending from a depth of 50 to lOO fathoms, has for its Cteno- 



* " Maniud de Cnnchyliologie," Paris, 1S87. 



