BRYOZOA. 99 



Introduction.] 



LUNARIUM: A more or less thickened portion of the posterior wall in many 

 paleozoic Bryozoa, which is curved to a shorter radius and usually projects above 

 the plane of the zooacial aperture. It is of crescentic form, and generally a con- 

 spicuous feature in tangential sections. 



PRIMARY APERTURE : "The original orifice " of the zooecium in the Cryptosto- 

 mata. 



SUPERFICIAL APERTURE : The outer orifice of the tubular prolongation (vestibule) 

 of the original aperture. 



OBVERSE and REVERSE : Two terms employed to designate, respectively, the 

 celluliferous and non-celluliferous faces of the zoaria of the Fenestellidce, Acantho- 

 cladiida', and Phylloporinidw. 



DISSEPIMENTS : Short non-celluliferous bars connecting the cell bearing branches 

 in the Fenestellidce, at short and regular intervals. The rounded, hexagonal, or quad- 

 rate meshes of the network thus formed are known as the " fenestrules." 



The following brief remarks upon the preservation, methods of study, classifica- 

 tion, and geological distribution of fossil Bryozoa, the paleozoic forms of America in 

 particular, may be of assistance to students. A more comprehensive general dis- 

 cussion of the subject is to be found in the introduction to my recent work in the 

 eighth volume of the reports of the Geological Survey of Illinois. 



PRESERVATION. 



It is evident that the hard parts of the Bryozoa only could have been preserved 

 in the fossil state. Equally obvious is the fact that these parts could consist only of 

 the outer investment of the polypides. The opportunities of the paleontologist are 

 restricted farther to those in which this investment was calcareous, or corneo-calca- 

 reous. Judging from recent conditions, it would appear that of by far the greater 

 part of the extinct forms, the colonies or zoaria were capable of preservation, since 

 in a very large proportion of the living marine Bryozoa the skeleton is calcareous. 



Certain changes in the composition and structure of the zoaria have always 

 accompanied the process of fossilization. Indeed, it is probable that the mineral con- 

 stituents of all fossils are never the same as they were in the living state. The least, 

 and I am glad to state, the commonest alteration is where the originally amorphous 

 calcite has been changed into the crystalline form of that mineral. In most cases 

 this change has been so gradual, and the crystals formed so minutely, that very little 

 of the structure has suffered obliteration. Very often many of the minutest details 

 are still to be recognized. This favorable condition prevails among the majority of 

 fossil Bryozoa, and is especially remarkable among those derived from Lower Silurian 



