364 NEW VORK STATE MUSEUM 



D. foliaceus and the observation of the occurrence of yreat numbers of 

 specimens bearing the same character in separate shale horizons, the writer 

 has not been able to come to the view that this form should be brought as 

 a mutation under D. foliaceus, still less that it should be united with 

 one of the varieties of D. foliaceus, as p. e. D. foliaceus var. 

 incisus. From the latter it clearly differs in the much closer arrange- 

 ment of the thecae (13-14 in 10 as against 8-9). In this regard it stands 

 much nearer to D. foliaceus var. acutus with 1 2 to 13 thecae in the 

 same space, to which variety it also is nearest related by the outline and 

 dimensions of the rhabdosome and form of thecae in compressed state ; 

 but from which it still differs in not showing any sutural groove on the 

 lateral face and possessing the concavo-convex section of the rhabdosome 

 and curvature of the thecae towards the reverse side. This latter pecu- 

 liarity, for which an explanation has been suggested on p. 98, it has in com- 

 mon with C 1 i m a c o g r a p t u s typical is. As long as this feature 

 has not been observed in D. foliaceus and its varieties, it seems to serve 

 better the purposes of taxonomy to keep this form under a separate specific 

 designation. 



There exists considerable variation in the width of the rhabdosomes of 

 the limestone material as a comparison of figures 302 and 304 will prove. 

 The virgella and lateral spines are but weakly developed. 



The appearance of the shale specimens is best illustrated by the mate- 

 rial from Bakers falls, where they occur in surprising multitude, clearly 

 forming a distinct zone of more than 50 feet thickness. The rhabdosomes 

 are all preserved in a whitish Giimbelitlike substance and extremely com- 

 pressed. They are mostly of the rugosus facies and present a striking 

 appearance by the uniformity of their widths, the small, closely arranged 

 "denticles" and broad unindented middle part [see 305 |, the little develop- 

 ment of appendages of the sicular end and the absence of a free nema- 

 caulus, by all of which characters they are easily distinguished from 

 the varieties of I), foliaceus in like shale preservation. There are 

 forms in the succeeding formations (Utica and Lorraine), which possess 



