INTRODUCTORY OBSERVATIONS. 3 



extreme limits of the Arctic to those of the Antarctic Ocean. They are found also at all 

 depths from above low-water mark to more than 100 fathoms deep. As regards geo- 

 graphical distribution, it may be remarked that, whilst many species appear to be uni- 

 versally distributed over the surface of the globe, many marine genera are confined to 

 either the austral or the boreal hemisphere ; and that one entire order, including the Fresh- 

 water Polyzoa, is, so far as we know, limited to a portion of the North Temperate 

 Zone. 



With this wide though unequal geographical range and great apparent adaptability to 

 external conditions, we may expect to find that the Polyzoa are of considerable importance 

 in a geological point of view. Their remains are found accordingly in rocks of nearly 

 every epoch, from the Paleozoic to the most recent. Though not numerous in species, 

 they are plentiful in the Upper Silurian and Carboniferous Limestones ; very abundant in 

 the Jurassic and Cretaceous rocks, in some of which, as in Le Cotentin, the Departments 

 of La Sarthe, Charente, &c., at Maastricht and Pauquemont, they appear even to 

 constitute the principal bulk of the deposit. In fact, speaking geologically, the Cretaceous 

 period seems to have formed the culminating point of the calcareous Polyzoa. But in 

 the tertiary rocks, also, of diflferent periods, their remains are nearly equally abundant, and 

 more especially is this the case in the Miocene deposits of Brittany and Touraine ; in 

 Hesse Cassel ; in the tertiary basin of Vienna ; and also in North America, in New Jersey 

 and Virginia ; whilst their abundance in the older Pliocene deposits of Suffolk and Norfolk 

 is evidenced in the present monograph. 



As regards the general conditions under which the Polyzoa which occur in a fossil 

 state have lived, M. D'Orbigny attempts to show, with every probability, that those most 

 conducive to their multiplication, are : — 1, a considerable depth of water, as is proved by 

 their occurring in association with the remains oi Fentacrinus and of Brachiojjoda; 2, clear- 

 ness or limpidity of the water; and 3, the existence of strong currents. But it should be 

 remarked that these observations are true only of certain forms of Polyzoa, and of those 

 chiefly which belong to the families or genera whose representatives or extinct allies are 

 found in the fossil sstate. Por it is well known that many species of a more fragile nature 

 occur, as before said, even above low-water mark, and that some especially delight in shallow, 

 muddy water. And this serves partly to explain a circumstance at first sight calculated 

 to excite some surprise ; namely, that whilst many recent genera, and even some recent 

 species, belonging to certain divisions of the class, or to certain families, frequently occur in 

 the fossil state, others which are known to exist under similar conditions at the present time, 

 and are far more numerous, are wholly wanting. But very little consideration will show that 

 the apparent deficiency at former periods does not necessarily prove the non-existence then 

 of similar or of representative forms. It is obviously due to the circumstance that several 

 entire orders, and many genera, have been unable to resist the attacks of external agencies ; 

 the inability arising either from the individual elements, though themselves sufficiently 

 calcareous, being so joined together by perishable connexions as to have fallen entirely 



