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NEW YORK STATE MUSEUM 



theca from the first and its crossing to the other side of the rhabdosome, 

 are all observable in favorably preserved shale material of D . f o 1 i a c e u s 

 [rig'. 286, 287] and other species. The sicula is seen to be furnished in most 

 species with a long needlelike virgella and an opposite short mucro. The 

 "lateral spines" which are also frequently developed into powerful spurs 

 [see fig. 286] are apertural spines of the two primary thecae [see p. 72 of 

 introduction]. In typical Diplograpti they are restricted to these two thecae. 



We have here excluded the spinose forms from Diplograptus for the 

 reason set forth in the introduction [p. 85] that with the development of 

 spinosity changes in the structure of the perisarc take place which are of 

 generic value. All spinose forms are here united under Glossograptus and, 

 for practical reasons, even such forms are brought under that caption in 

 which the perisarcal structures are not quite apparent, as G . (?) eucharis. 

 Thus restricted the genus Diplograptus comprises, in the faunas here 

 described, only D. foliaceus with its many varieties and mutations 

 (var. acutus, alabamensis, incisus, trifidus and mut. ves- 

 pertinus), D. crassitestus, D. amplexicaulis (and var. per- 

 tenuis), D . a n g u s t i f o 1 i u s, D . euglyphus and D . p e o s t a. 



In no genus of graptolites does one encounter greater difficulties in 

 distinguishing the species than in this. Three reasons can be readily cited 

 for this troublesome fact, namely the great differences in the aspects of the 

 flattened shale material and of the limestone and shale material preserved 

 in relief, the protean variety of aspects of the same form according to the 

 different directions of compression and the sides of the rhabdosome facing 

 the viewer and finally the multitude of varieties and mutations. These 

 combined produce such a confusing number of similar, but still distinct, 

 forms that one is tempted to distinguish but very few, widely varying 

 species, as has been done hitherto; but the fact that in certain layers of the 

 beds, with definite associations of graptolites, or in one locality but one 

 variety of the species is found, makes the differentiation of all these forms 

 imperative. The origin of the differing aspects of the rhabdosomes of 

 D. foliaceus has been set forth with great clearness by Professor Lap- 



