746 NEW YORK STATE MUSEUM 



graptolite beds, since our evidence goes to show that also here this fauna 

 precedes the Tetragraptus-Phyllograptus fauna. From his statement that 

 Didymograptus bifidus is very rare in the Bendigo series and " per- 

 liaps indicative of the higher beds of the series" I infer that in time 

 the division line corresponding to that between our Tetragraptus zone and 

 the zone with Didymograptus bifidus will be drawn through the 

 Bendigo series, and likewise a division line between the latter zone and that 

 of Dip log rapt us dentatus may be found in the Castlemaine series, 

 and a very exact correlation of the American and Australian zones will thus 

 be attained. At any rate so much is certain : that all the American zones 

 of the Lower Ordovicic are represented in Australia. The number of 

 forms common to America and Australia, which appear in Mr Hall's lists, is 

 truly astonishing and will rather increase, I believe, as investigation proceeds. 



Mr Hall adds to the few Lower Ordovicic forms cited by Freeh [1897J 

 from New Zealand Didymograptus bifidus, of which he has 

 examples from Nelson, N. Z. 



Note 2 On page 517 of this memoir allusion is made to the description 

 of an observation on the structure of the central disk of Dichograptus 

 octobrachiatus which has been omitted in its proper place. In the 

 example figured on plate 9, figure 1, the central disk has been split through 

 the median plane in the separation of the slabs. On one slab a strong car- 

 bonaceous (chitinous) test is shown, on the other a calcareous layer, which 

 rests on another carbonaceous test and clearly has segregated between the 

 two carbonaceous tests. This calcareous layer is thickest between the branches 

 and thins out toward the margin of the disk and upon the branches without, 

 however, becoming everywhere discontinuous along the median line of the 

 latter. The presence of this intercalated cake of carbonate of lime indicates 

 that there must have existed either a space open long enough within the 

 central disk to allow the segregation of the carbonate of lime, or at least an 

 original plane of separation between two tests, where the deposition of the 

 calcareous salt could take place. The form of the calcareous layer is such as 

 to suggest the presence of an original open space in the disk, but at any 

 rate the occurrence indicates the composition of the central disk of two tests. 



