Affinities of the Genus Parkeria, Carp. 7 



■vvliicli are separated hy interspaces broken up into irregular 

 " chamberlets " (see woodcut). One of the most remarkable 

 points about Parheria is the structure of the tissue which com- 

 poses the radial pillars and concentric lamellae. The nature of 

 this tissue can be admirably studied in thin sections of calca- 

 reous examples of Parkeria (PI. III. figs. 1-4), which show- 

 that it is composed of innumerable, minute, cylindrical or 

 polygonal tubuli, which have a radial disposition and are 

 united to one another by their walls. The walls of the tubuli 

 are porous, thus allowing of a free communication between 

 the cavities of adjacent tubuli. Vertical sections (PI. III. 

 figs. 2 and 4) show that the tubuli are vertical to the surface 

 in the axes of the radial pillars, but that they become oblique 

 to the surface as they proceed upwards. Hence each indi- 

 vidual tubule has a proportionately short vertical course, 

 beginning in the axis of the pillar and then gradually bending 

 outwards so as to open obliquely on the circumference of the 

 pillar. In cross sections of the pillars (PI. III. figs. 1 and 3) 

 the tubuli in the centre of the pillars are cut at right angles 

 to their course and therefore present themselves as a poly- 

 gonal network, like that of a Monticuliporoid. On the other 

 hand, the tubuli in the peripheral portions of a cross section 

 through a pillar are necessarily cut more or less obliquely, 

 and their mouths are shown at the actual circumference of 

 such a section. In the concentric lamella which connect 

 contiguous pillars the tubuli are necessarily short ; they 

 originate from the calcareous membrane which forms tlie 

 inferior surface of the lamella, and they open by minute 

 apertures superiorly either into the cavities of the " chamber- 

 lets " or, in the case of the last-formed lamella, upon the 

 outer surface of the coenosteum. 



The diameter of the tubuli composing the general cancel- 

 lated tissue of Parkeria is about ^rg- millim. The tubuli 

 exhibit nothing of the nature of radiating septa, nor do they 

 possess any "tabula3" or transverse internal partitions. 

 Owing to the fact that the tubuli are slightly tiexuous, and 

 owing also to their very minute size, longitudinal sections 

 rarely divide a tubule in a single plane, but pass from side to 

 side of its middle line, allowing the porous wall of the tubule 

 to be seen at intervals, or here and there actually cutting the 

 wall. Hence in longitudinal sections (PI. III. tigs. 2 and 4) 

 there is often an appearance of tabula ; but this appearance 

 is due to the cause just mentioned. In the same Way, sections 

 which cut the tubules obliquely give the appearance not of 

 proper tubes, but of rows of elongated polygonal cells. The 

 general cancellated tissue of Parkeria may be regarded as 



