2 Prof. H. A. Nicholson on the Structure and 



Mr. Carter were based essentially upon specimens of ParJceria 

 in one state of preservation, viz. specimens in which the 

 chambers are non-infiltrated and the skeleton largely com- 

 posed of phosphate of lime. The beautiful illustrations which 

 accompany Dr. Carpenter's memoir were similarly derived 

 mainly from specimens in the above condition of minerali- 

 zation. This condition was regarded by Dr. Carpenter as 

 being the one most nearly representing the original constitu- 

 tion of the fossil, whereas Mr. Carter recognized that it was 

 secondary and the result of changes produced during the 

 process of fossilization or at some subsequent period. The 

 correctness of Mr. Carter's conclusion on this point does not, 

 m my opinion, admit of reasonable doubt ; and I propose in 

 what follows to briefly describe the structure of Parheria as 

 exhibited by what I regard as normal examples of the genus, 

 viz. examples in which the skeleton is composed of non- 

 crystalline carbonate of lime and the chambers are infiltrated 

 with calcite. Specimens in this condition of preservation are 

 by no means uncommon ; the phenomena which they present 

 differ in no respect from those shown by similarly preserved 

 examples of other calcareous fossils, such as corals, Polyzoa, 

 Stromatoporoids, &c. ; and their internal structure is so admi- 

 rably preserved that they can be readily studied by means of 

 thin sections. For much of the material with which I have 

 worked I am indebted to the generosity of my friend Prof. 

 T. M'Kenny Hughes, who was good enough to present to 

 me a large series of specimens in all states of preservation. 

 1 have also to express my gratitude to Dr. P. Herbert Car- 

 penter, who furnished me with the opportunity of examining 

 the beautiful and instructive series of preparations of Parkeria 

 upon which Dr. W. B. Carpenter had based his original 

 memoir. Lastly, I have to thank Dr. John Millar for the 

 free use of a large and interesting series of thin sections of 

 Parkeria. My observations, however, are in the main founded 

 upon an extensive series of slides which I have prepared 

 myself. 



1. General Form and Mode of Growth. 

 The ordinary form of Parkeria [P. sphcerica) presents itself, 

 as is well known, in the shape of spherical bodies, which 

 vary in diameter from less than lialf an inch to about two 

 inches. The surface in unworn specimens exhibits rounded 

 or elongated elevations, separated by intervening depressions, 

 or, in other instances, may present an alveolar or honey- 

 combed aspect. Rough fractures show that the skeleton is 

 composed of numerous cylindrical columns (" radial pillars "), 



