Affinities of the Genus Parkeria, Carp. 9 



calcareous membrane, which was probably always at first 

 attached by its lower surface to some foreign body, such as a 

 fragment of shell, though this latter seems to have been com- 

 monly absorbed in process of growth. The upper or free 

 surface of the primordial lamina supports a layer of the cha- 

 racteristic tubulated tissue of Paj-keria, which is prolonged 

 upwards at small intervals into long pointed spines. This 

 condition can be studied in vertical sections traversing the 

 centre of the skeleton, and at this stage of its existence the 

 organism must have presented a close resemblance to the 

 crust of Hydractinia echinata, allowing for the fact that the 

 coenosteum is calcareous and not horny. In the further pro- 

 gress of growth the spines or primordial radial pillars throw 

 out from their summits lateral outgrowths, which coalesce 

 with one another more or less completely, and thus give rise 

 to a second lamina, separated from the original one by an 

 intervening space. This second lamina agrees with the first 

 in having a calcareous membrane below and in being com- 

 posed of tubulated tissue prolonged upwards into spines, which 

 are mostly radial continuations of the primitive spines. The 

 interspace between the first and second lamina is necessarily 

 crossed by the primitive radial pillars, and hence appears in 

 section as if broken up into separate " chamberlets " (PI. III. 

 fig. 6). As development proceeds the spines of the second 

 series produce again a third lamella, thus giving rise to a 

 second series of chamberlets ; and so the process goes on till the 

 organism has attained its full growth. The later lamella are 

 generally thicker than the earlier ones and the rows of cham- 

 berlets proportionately more contracted (woodcut, p. 6) ; but 

 there is no reason to think that the former are produced in any 

 way differently from the earlier ones. After a certain number 

 of lamellffi, with their corresponding rows of chamberlets, have 

 been formed, it is common to find that a thin concentric layer 

 of coarse cancellated tissue is produced, this possibly repre- 

 senting, as before suggested, a stage in the life of the organism 

 in which reproductive zooids were developed. Then the ordi- 

 nary lamellse and spines are again formed, and we have a new 

 series of concentric lamellce and chamberlets. It is also 

 not uncommon to find that growth is at first unilateral, the 

 first two or three lamellae being on one side only of the pri- 

 mordial crust, while the later lamellai form complete concentric 

 envelopes round the first-formed portion of the organism. 

 It follows from the above account of the mode of develop- 

 ment of the skeleton in Parheria that each successive tier of 

 chamberlets necessarily represents what was at one time the 

 surface of the organism. 



