The difficulty of assigning special genera or species to special ‘zones of depth’ is increased by two important facts in the phenomena of distribution. In the first place, it is found that species which occur in shallow water in northern seas often extend to very deep water in much lower latitudes. This interesting fact, which shows the importance of temperature in determining distribution, was first established by the dredgings of the _Lightning_ and _Porcupine_ off the western coasts of Europe. In the second place, a certain number of species seem equally at home in shallow and in abyssal waters, in cases where a great difference of latitude does not occur to equalise the temperature. Thus the _Challenger_ found _Venus mesodesma_ living on the beach (New Zealand) and at 1000 fath. (Tristan da Cunha); _Lima multicostata_ in ‘shallow water’ (Tonga and Port Jackson) and at 1075 fath. (Bermuda); _Scalaria acus_ from 49 to 1254 fath. (N. Atlantic); and _S. hellenica_ from 40 to 1260 fath. (Canaries). The _Lightning_ and _Porcupine_ found, or record as found,[382] _Anomia ephippium_ at 0 to 1450 fath., _Pecten groenlandicus_ at 5 to 1785 fath., _Lima subauriculata_ at 10 to 1785 fath., _Modiolaria discors_ at 0 to 1785 fath., _Crenella decussata_ at 0 to 1750 fath., _Dacrydium vitreum_ at 30 to 2750 fath., _Arca glacialis_ at 25 to 1620 fath., _Astarte compressa_ at 3 to 2000 fath., and _Scrobicularia longicallus_ at 20 to 2435 fath. _Puncturella noachina_ has been found at 20 to 1095 fath., _Natica groenlandica_ at 2 to 1290 fath., _Rissoa tenuisculpta_ at 25 to 1095 fath. In many of these cases we are assured that no appreciable difference can be detected between specimens from the two extremes of depth. In spite, however, of these remarkable vagaries on the part of certain species, we are enabled roughly to distinguish a large number of genera as ‘shallow-water’ and ‘deep-water’ respectively, while a still larger number occupy an intermediate position. Among shallow-water genera may be named _Patella_, _Littorina_, _Nassa_, _Purpura_, _Strombus_, _Haliotis_, _Mytilus_, _Cardium_, _Solen_; while among deep-water genera are _Pleurotoma_, _Scissurella_, _Seguenzia_, _Dentalium_, _Cadulus_, _Limopsis_, _Nucula_, _Leda_, _Lima_, and _Axinus_.