500 



SCIENCE. 



[N. S. Vol. II. No. 42. 



eating genetic relationship being the ' angle 

 of divergence.' 



These views are illustrated by reference 

 to forms belonging to the ' genera ' Bryo- 

 graptus, Diehograptus, Tetragraptus and Didy- 

 mograptm, which appear in turn in this 

 sequence. 



Out of nine Tetragrapti (and the authors 

 know of no other forms referred to this 

 genus which are represented by well-pre- 

 served examples), eight are closely repre- 

 sented by forms of Didymograptus, which are 

 closely comparable with them as regards 

 characters of hydrothecse and amount of 

 ' angle of divergence,' whilst the ninth is 

 comparable with a Didymograptus as regards 

 ' angle of divergence ' only. Moreover, four 

 of the Tetragrapti are comparable as regards 

 the two above-named important characters 

 with forms of Diehograptus and Bryograptus 

 with eight or more branches, and the 

 authors confidently predict the discovery of 

 forms belonging to these or closely allied 

 many-branched ' genera,' agreeing with the 

 remaining Tetragrapti in what they regard 

 as essential characters. 



They give details showing the points of 

 agreement of each group of the various 

 series, including a two-branched, a four- 

 branched, and a many-branched form, and 

 point out how diificult it is to understand 

 how the extraordinary resemblances be- 

 tween the various species of Tetragraptus and 

 Didymograptus (to take one example) have 

 arisen, if, as usually supposed, all the 

 species of the genera have descended from 

 a common ancestral form for each genus, in 

 the one case four-branched, and in the other 

 case two-branched. On the other hand, it 

 is comparatively easy to explain the more 

 or less simultaneous existence of forms pos- 

 sessing the same number of stipes, but 

 otherwise only distantly related, if they are 

 imagined to be the result of the convergent 

 variation of a number of different ancestral 

 types. They allude to similar phenomena 



which have been shown to exist amongst 

 other organisms; thus Mojsisivics has de- 

 cribed analogous cases amongst the Am- 

 monites, and Buckman (under the name of 

 heterogenetic homceomorphy) amongst the 

 brachiopods, though in this instance the 

 cases of ' species ' and not of ' genera ' are 

 considered. 



Following the above inferences to their 

 legitimate conclusion, the authors point out 

 how ' genera ' like Diplograptus and Mono- 

 graptus may contain representatives of more 

 than one ' family ' of graptolites, according 

 to the classification now in vogue, which 

 would account for the great diversity in the 

 characters of the monograptid hydrothecse. 



In conclusion, the authors offer a few 

 theoretical observations upon a possible 

 reason for the changes which they have 

 discussed in the paper. 



The latter of the foregoing authors, with 

 Mr. E. J. Garwood, also read an important 

 paper on the zoning of the Carboniferous 

 rocks which they had begun in the north 

 of England. The zones so far established 

 were the following : 



Zone of Productus c. f. edelburgensis. 



Zone of Productus latissimus. 



Zone of Productus giganteus. 



Zone of Chonetes papilionaeea. 



Zone of Spinfera octoplicata. 



Mr. Garwood has traced the zone of P. 

 latissimus, occupying the same relative posi- 

 tion to that of P. giganteus, from Settle, in 

 Yorkshire, to the Northumbrian coast, near 

 Howick Burn. 



With regard to the other papers coming 

 under this heading it will suffice to mention 

 that the attempt to obtain the rest of the 

 skeleton of the Oxford Cetiosaurus has not as 

 yet met with any success, and that the re- 

 port on fossil Phjdlopoda by Professor 

 Eupert Jones contained a most valuable 

 table by Professor Lapworth on the distri- 

 bution of these organisms. The discovery 

 of a new section of Ehatic rocks was de- 



