354 



MOLLUSCOIDEA— BRYOZOA 



PHYLUM V 



portions : iiiore or less erect and very irregiüaiiy disposed. Orifice terminal, entire 

 or sinuated, witli or wifhoiit internal denticles ; in connection witli it are usually one 



or more rostra bearing avicularia. Inter- 

 calated avicularia generally present also. The 

 surface of weatliered specimens dotted by 

 the unequal apertnres of vesicle-like cells. 

 Tertiary and Recent. 



Subclass 2. PHYLAOTOLAEMATA 

 AUman. 



The Bryozoa referred to this subclass 

 are soft - bodied, and therefore cannot be 

 expected to be found fossil except under 

 unusual conditions of preservation. The 

 fresh- water Cenomanian beds of Bohemia 

 have yielded an organism incrusting a Unio, 

 resembling the Recent genus Plumatella. 

 Although the structure is too iniperfectly preserved for certain Identification, this 

 specinien {Plumatellites proliferus Fric) may well be a fossil representative of the 

 Phylactolaemata. 



Fig. 525. 



Cellepora conglomerata Goldfuss. Oligocene 

 Astrupp, near Osnabrück. A, Zoarium, i/i 

 B, Upper surface, enlarged. 



Range and Distribution of the Bryozoa. 



The class Bryozoa begins in the earliest OidaYifiianj and is represented con- 

 tinuously up to the present time. The older Paleozoic forms belong chiefly to two 

 Orders — the Cyclostomata and Trepostomata. 



A considerable nuinber of Cyclostomatous genera are present in the Ordovician, 

 niost of them being closely related with Mesozoic and Recent types ; but throughout 

 the remainder of the Paleozoic, and in the Trias also, the order is very sparingly 

 represented (except for the Ceramoporidae and Fistuliporidae), and in some parts quite 

 absent. In the Jura and Cretaceous, however, a reniarkable increase took place, 

 hundreds of species being known from these forniations. During the Tertiary their 

 strength was again materially reduced, and the living Cyclostomata barely exceed 

 100 species in number. 



The Trepostomata appear suddenly and in great variety in the Ordovician, from 

 which over 400 species are known, but entered almost immediately upon a period of 

 decline. From the Trenton and Cincinnati groups alone more species have been 

 described than from all of the later Paleozoic formations put together. There is at 

 present no evidence to show that the group survived later than the Paleozoic era, but 

 it is not unlikely that their descendants may be found among certain Mesozoic 

 families, such as the Ceidae, which are provisionally assigned to the Cyclostomata. 



The Cryptostomata are likewise confined to rocks of Paleozoic age, but, as has 

 been remarked above, may be very confidently regarded as the forerunners of the 

 Cheilostomata, True members of tlie latter group are first met with in the Jura, but 

 they develop rapidly, and from the Cretaceous onward remain the dominant type. 



The Triassic and Liassic Bryozoans belong chiefly to the Cerioporidae. This 

 family, together with the Diastoporidae, Fascigeridae, and other members of the 

 Cyclostomata, is abundantly represented in the Middle Jura of Lorraine, Southern 

 Germany, England and Normandy. The Upper Jura, on the contrary, yields com- 

 paratively few Bryozoan fossils. 



The Cyclostomata still predominate in the Neocomian and Gault, but in the 



