IOO NEW YORK STATE MUSEUM 



effect is much more striking than with any other group of fossils. Xot 

 only does a beginner believe upon superficial examination in the specific 

 distinctness of two different preservation facies of the same form but experi- 

 enced observers have several times, after careful stud) - , committed the same 

 error. The most striking instance of this is Diplograptus f o 1 i a c e u s 

 Murchison, to which many so called species are to be assimilated. The 

 following remarks will have special reference to this species, although more 

 or less applicable to the graptolites generally. 



To begin with the true characters of a Diplograptid polypary are only 

 seen in the uncompressed state, i. e. when the thecae have been filled out 

 with mineral matter so that they do not, upon compression, flatten out to a 

 thin film. In this state D. foliaceus has been long known in America 

 under the name of D. amplexicaulis Hall of the Trenton limestone. 

 Professor Hall's figures and description refer, not to the complete polypary 

 (which apparently he did not possess) but to the impression left upon the 

 limestone after the removal of the complete polypary. Thus in addition to 

 the exact and detailed agreement between such imprints and Professor 

 Hall's figures we have his statement that the fossil forms a thin film upon 

 the rock, and the further fact that neither his figures nor this description 

 could possibly refer to the complete polypary. This preservation facies, 

 which is the usual one in the limestone and becomes common in the shale 

 in proportion as they are calcareous, I shall throughout this paper designate 

 as the normal facies. It is so well described in the extract from Professor 

 Lapworth's manuscript report 1 that it requires no further discussion. 



Such a polypary may be compressed in several directions. Some one 

 of these preservation facies is the usual condition found in soft shales ; and 

 as these are the principal graptolite-bearing beds these fossils are generally 

 seen as flattened films. 



A cross-section of a generalized diplograptid polypary may be diagram- 

 matically represented as a rounded oblong. This polypary may undergo 

 compression in three principal ways; viz (i) along the longer diameter, 

 (2) along the shorter diameter and (3) along the diagonal. The first pro- 

 duces an ordinary bilaterally symmetrical view, which I shall name the direct 

 facies; the second the " scalariform " facies and the third a facies which 

 may be called the oblique facies. The last is well shown by Hmmons's D. 

 d i s s i m i 1 a r i s. A little thought will show that in this form the thecae 

 must overlap on one side and underlap on the other. 



When a form like Diplograpt u s a m plexicaulis imbedded in 

 the soft mud is subjected to compression those parts of the polypary which 

 are least rigidly connected with the common bod)- and virgula suffer the 

 greatest amount of displacement relative thereto. It is evident that this 



1 See under Diplograptus. 



