206 DE. W. D. LANG ON [vol. lxxiv, 



It has been claimed l that the Cheilostome Polyzoa, in evolving 

 from primitive to specialized forms, continually pass from a less to 

 a more calcareous condition, and that, consequently, their skeletons 

 contain more and more calcium carbonate as any one line of 

 evolution proceeds. Further, it Avas claimed that this secretion 

 of calcium carbonate (whether originating as a by-product of 

 metabolism, or as the final result of an explicit excretory act), 

 though at first, possibly, of use to the organism for rendering its 

 skeleton more protective, very soon got out of control, probably 

 through the inefficiency of inhibiting factors ; and it became a 

 pressing problem in the organism's bionomy Avhere to dispose of its 

 increasing superfluity of calcium carbonate so as least to interfere 

 with its normal functions. There is no evidence that the Polyzoa 

 ever gained control over this derangement of metabolism, or that 

 they ever learned to counter it by resorption ; but a good deal to 

 show that these calcareous lineages are doomed to ultimate extinction 

 under their masses of superabundant skeletal tissue. 



III. The Kelestominae. 



It is to be expected, then, that the dominant feature in the 

 development of the Kelestominae from the Primitive Pelmatoporid 

 will be the greater calcification of the skeleton. It has been 

 shown l that this is brought about in the cribrimorphs mainly by a 

 general solidification of the intraterminal front-wall, and a deposi- 

 tion of secondary calcium carbonate on the middle portion of that 

 structure ; by laying down secondary calcium carbonate in the 

 intercecial valleys ; and by building up secondary structures in the 

 neighbourhood of the aperture. All three methods are used in 

 the Kelestominae, but the last two are the most prominent. Be- 

 sides these three general methods of depositing secondary calcium 

 •carbonate, the following structural modifications occur in the 

 Pehnatoporidae generally, many of them ministering to one or 

 other of the general methods, or affording by themselves the 

 •opportunity of increased deposition of calcium carbonate. (Cata- 

 genesis, however, often implying a smaller deposit of calcium 

 carbonate in the structure concerned, may come about in certain 

 •cases. ) The asty tends to become erect and bilaminar or cylindrical ; 

 the cecium tends to increase in size, to become shorter compared 

 with its length, and more parallel- sided ; the number of costae 

 increases, but may again diminish by catagenesis ; the apertural 

 spines are reduced from six to four, but are often greatly enlarged 

 and otherwise modified in connexion with a secondary aperture ; 

 the avicularia become fewer, definitely distributed, definitely 

 •orientated, dimorphic or polymorphic, larger, and with more 

 pointed apertures. 



1 W. D. Lang, Geol. Mag. dec. 6, vol. iii (1916) pp. 74-76, and Ann. & Mag. 

 Nat. Hist. ser. 8, vol. xviii (1916) p. 82. 



