Affinities of the Genus Parkeria, Carp. 3 



which pass in a radiating manner from the centre to the 

 circumference of the fossil, and are united at more or less 

 regular intervals by imperfect concentric layers, which are 

 separated from one another by concentrically disposed inter- 

 spaces (" chamberlets "). 



As regards the mode of growth, most specimens must have 

 been entirely free in the adult condition, since the entire 

 surface is covered equally with the ends of the radial columns. 

 Some examples, however, are pierced by a central cylindrical 

 canal, as if they had grown upon some such body as the stalk 

 of a Crinoid, which had been subsequently dissolved out. In 

 other cases there is a similar central canal, but this does not 

 extend more than halfway through the fossil. A similar 

 complete or incomplete central perforation is common in the 

 genus PorospJicera^ Steinm., though my examination of this 

 latter genus would lead me to conclude that it is in no way 

 related to Parkeria. In most of the examples of Parheria 

 which I have examined, I have been unable to detect in the 

 centre of the fossil any foreign body round which the organism 

 may have grown. In this respect most examples resemble 

 the singular fossil which I have recently described as Mitchel- 

 deania gregaria. In some cases, however, the innermost 

 or primordial layer of Parkeria has unquestionably been 

 attaclied to the exterior of a foreign body, which appears 

 generally to have been the shell of a small Nautilus or Ammo- 

 nite. I have figured (PI. III. fig. 6) a vertical section across 

 the centre of such a specimen, showing the chambered Cepha- 

 lopod round which the Parkeria has grown in successive 

 concentric layers. It does not appear to me to admit of doubt 

 that the chambered " nucleus " ascribed to Parkeria by 

 Dr. W. B. Carpenter is really a foreign body of the above 

 nature. 



2. T'he Chemical Constitution of the Skeleton. 



If any large series of specimens of Parkeria be examined, 

 it will be found that different examples present great differ- 

 ences as regards the mineral nature and composition of the 

 skeleton. The following are the principal variations which 

 may be recognized in this respect : — 



{a) In a certain proportion of specimens the skeleton is 

 composed of carbonate of lime and the chambers of the fossil 

 are occupied by calcite or by an infilling of the matrix in 

 which the fossil was originally imbedded. The skeleton- 

 fibre in these specimens is not in the condition of ordinary 

 calcite, but is composed of subcrystalline granules of carbonate 

 of lime. 



1* 



