W. D. Lang — The Reptant Eleid Pohjzoa. 65 



separated by thin, low ridges. They are immersed in the zoariura. 

 Occasionally parts of the zoarium occur, free from apertures, but 

 divided into irregular polygonal areas by thin, low ridges (see Fig. 12). 

 That species of reptant Eleid which this form most resembles is 

 Beptoceritites JRoioei, Gregory, from the Lower Senonian of Chatham. 

 The three obvious points of difference are : {!) B. Bowei has an 

 unilaminar zoarium ; (2) it has avicularia ; (3) it has no closed 

 zooecia. The systematic value of these points will be discussed 

 later. On the strength of the first two it is placed in the genus 

 Beptoceritites. The chief zooecial difference between the two species 

 lies in the fact that the zooecia of Semimultelea Dixoni are immersed, 

 while those of Beptoceritites Bowei are very emergent. The 

 specimen was obtained from the Lower Senonian, zone of Micraster 

 cor-testudinarium, from the chalk-pit opposite the Rose and Crown 

 Inn, Kenley, south of Croydon, Surrey. The pit is No. 32 of 

 Dibley ^ and No. 113 of Young.^ 



IIL The Validity of the ' Genera.' 



The table given on pp. 62-3 shows the principles on which the Eleid 

 genera are at present based. Nor are those principles confined to 

 the Eleidfe ; they are fundamental in all the families of Cyclostomata. 

 If the systematic arrangement of forms is to be finally established 

 on an evolutionary basis, and the term ' genus ' to be used to include 

 those species whose nearest common ancestor differed from that of 

 another group of species, the ' genera ' as at present established in 

 the Cyclostomes and Eleids are obviously artificial, for they represent 

 groups of species of various genera in the same stage of phylogeny, 

 and are such as would be the case if, for instance, a given genus of 

 Ammonites included all forms with a keel. Gregory ^ has realised 

 how artificial are the ' genera ' in the Cyclostomes, but advocates their 

 retention on the plea that they are " convenient " though " artificial 

 groups of species." But he evidently did not allow that the phylogeny 

 of the group could be found in the astogeny * of the colony, for he 

 claims that the Bradford Clay Bereniceas were the direct descendants 

 of the Great Oolite Diastoporas. If once the astogenetic principle is 

 granted the task of establishing new genera, though one requiring 

 much material collected rigidly according to exact horizon, is not as 

 hopeless as he describes ; still less in the Eleids where more zooecial 

 characters are available than in the Cyclostomes. 



On reference to the table on pp. 62-3, which gives the specific 

 distinctions in this family, it will be noticed that in regard to 



1 G. E. Dibley, "Zonal Features of the Chalk Pits": Geol. Assoc. Proc, 

 vol. xvi, pt. 9 (1900), pp. 490-491. 



2 G. W. Young, "The Chalk Area of North-East Surrey": Geol. Assoc. Proc. 

 (1905), p. 206. 



3 J. W. Gregory : British Museum Catalogue of Jurassic Polyzoa, 1896, pp. 14-22. 



* Cumings' term for the ontogeny of a colony. See E. E.. Cumings, "Develop- 

 ment of Fenestella'" : Amer. Journ. Sci., vol. xx (1905), footnote to p. 169. In 

 a colonial organism like a Polyzoan, three developments have to be considered : 

 (1) ontogeny, the development of the individual; (2) astogeny, the development of 

 the colony ; (3) phylogeny, the development of the race. 



