542 NEW YORK STATE MUSEUM 



the living animal, no longer determinable, as is pointed out by Frecb \loc. cit. 

 p.551j. In reference to this last suggestion, it is however to be remembered 

 that the calcite layer, though of i-emarkably uniform thickness, is not always 

 found on the same side of the carbonaceous periderm and, though mainly 

 developed on the inside, may also occur on the outside or even on both sides 

 \see fig. 14] ; and that, further, the calcite band is often directly continuous 

 with the numerous small calcite veins transecting the pyrite. The latter fact 

 would suggest that the calcite band may have been formed after the harden- 

 ing of the organic material which caused the deposition of the pyrite. That 

 the latter filled the rhabdosomes at a very early stage of the fossilization, is 

 attested by the failure of these rhabdosomes to beconie compressed. On the 

 whole, the calcite bands in the Deep kill material appear to have filled the 

 interspaces between the periderm and the organic or pyrite matrix, arising 

 from a shrinkage of the latter. 



The presence of an epidermic layer which, according to Wiman's observa- 

 tions, appears to have had its independent system of growth lines, seems 

 important in so far as it would indicate that the periderm was not an external 

 skeleton, but was formed in the mesoderm. The latter conclusion has urged 

 itself on the writer ever since he studied the colonies of Diplograptus, by 

 various arguments, the most important of which are the following : 



1 The nema or nemacaulus of Diplograptus ^ristis Hall (f o 1 i - 

 a c e o u s Murchison) shows from the sicula onward to the perfection of 

 the first or primary rhabdosome [see Ruedemann, 1895, pl.3, fig.8-14] a con. 

 tinuous growth in length and thickness, which in a horn^^, nonporous body 

 can be explained only by the action of an external tissue. The secondary 

 growth of the nema is also specially remarkable in Tetragraptus 

 fruticosus [pl.lO, fig.7]. 



2 The stems of the larger multiramous dichograptids, as notably 

 Goniograptus [pl.6], which consist of thecae, continue to grow in thickness 

 till almost all traces of their thecal structure have disappeared. 



3 Certain species of dichograptids develop a secondary disk at the basis 

 of the rhabdosome [see Dichograptus octobrachiatus, pl.8, fig.4j. 



