Reports & Proceedings — The Royal Society. 41 



I. — The Eoyal Society. 



JVbvemier 22, 1917. — Sir J. J. Thomson, O.M., President, in the 



Chair. 



The following paper was read : " The Pelniatoporinse, a Group of 

 Cretaceous Polyzoa." By W. D. Lang, M.A., P.G.S. (Communicated 

 by Dr. r. A. Bather, F.R.S.) 



The evolution of this sub-family is considered in detail. In order 

 to present the facts intelligently, they are marshalled according to 

 the following theoretical considerations: — 



1. The species lie along diverging lineages ; towards the bases or 

 proximal ends of these are forms (radicals) with less calcareous 

 skeletal matter and less elaboration of structure, and these forms 

 appeared earlier in geological time ; towards their higher or distal 

 ends ai"e forms with more skeletal matter and more elaborate structure, 

 appearing later in geological time. 



2. The evolutionary tendency was to deposit the increasing 

 superfluity of calcium carbonate where it least interfered with the 

 organism's bionomics ; if possible, in such position and shape as 

 miglit even be useful to the organism. Sooner or later the race 

 perished through being unable to cope with its constitutional and 

 increasing habit of excessive secretion of calcium carbonate. 



3. There is a predisposition in radical forms of different lineages 

 to deposit their superfluous calcium carbonate along corresponding 

 tracts and with it to build up similar secondary sti'uctures. They 

 differ in compai'ative rate of building and in amount of elaboration, 

 as well as in details in architecture and ornament. Consequently, (a) 

 in most cases it is possible to predict the general history of a lineage 

 from an examination of one of its early terms ; and (J) lineages 

 often present series of homoeomorphic forms ; while characters 

 diagnostic of genera, and still more of congeneric lineages, often 

 appear trivial and of little importance to the organism. 



4. The "Law of Recapitulation " holds good in post-embryonic 

 growth-stages, not only of the individual but of the colony as 

 expressed by the individual constituents at successive distances from 

 its starting-point. In fossil Polyzoa, astogeny (as Cumings called 

 the development of the colony) is more easily observed than ontogeny. 



5. Periodicity is displayed by the Pelmatoporinse in their evolution. 

 The relations of tiie various forms are inferred from their adult 



morphology. Their stratigraphical distribution is considered in order 

 to confirm these relations, though the evidence of stratigraphy can 

 only be negative ; it can contradict a supposed relation, but cannot 

 affirm it. Finally, astogeny of forms is used to test morphological 

 results. 



Incidentally, results obtained by W. K. Spencer in his investiga- 

 tions on evolution of Cretaceous star-fishes are compared and found 

 generally to correspond with results described in this paper. 



