294 PALEONTOLOGY OF ILLINOIS. 



style of growth one large section can be made to show the 

 characters of the zocecia from their origin to the aperture. 



For massive, parasitic, or discoid zoaria, two sections (verti- 

 cal and transverse) will ordinarily suffice to bring out the 

 principal characters, but it is advised that two or more trans- 

 verse sections be prepared, dividing the zoarium at different 

 heights. 



I. Mode of growth. In considering the general structure of 

 fossil Bryozoa, the mode of growth and the form ultimately as- 

 sumed by the zoarium, though generally of but sub-ordinate classi- 

 ficatory value, may very appropriately be mentioned first. The 

 out ward form of the zoarium, though extremely variable, when the 

 whole class is taken into consideration, is tolerably constant for 

 each species, and not infrequently all or most of the species belong- 

 ing to one genus may adhere more or less strictly to some particu- 

 lar method of growth. The importance of variation in outer 

 form as a factor in classification is very much diminished by 

 the fact that many species may vary considerably in shape, and 

 but very slightly in their minute structural characters, while on 

 the other hand structurally diverse species may assume the 

 same outward form. We must admit, however, that in many 

 Bryozoa the ultimate form of the zoarium is largely determined 

 by the shape and reproduction of the zooficia, and in these the 

 external form is of the first importance, since it is indicative of 

 variations in structural characters. 



The zoaria of Palaeozoic Bryozoa usually exhibit one or the 

 other of the following conditions: 



1. The incrusting or parasitic zoa.rium, in which the colony 

 is parasitically attached by the whole of the lower surface to 

 foreign bodies, such as shells corals other Bryozoa or stones. 

 The zooecia are short, usually tubular, and by the formation of 

 successive layers the zoaria may form large masses. Examples 

 are several species of Leptotrypa, Spatiopora, Palseschara, 

 Atactopora, Ceramoporella, Eridopora, Stomatopora, etc. 



2. The laminar zoarium, differs from the encrusting only in 

 being free and in having the lower surface covered with a 

 wrinkled or striated epithecal membrane. The colony usually 

 commences its growth upon some small object. This type of 



