LOWER SILURIC SHALES OF THE MOHAWK VALLEY 



Si 



Fig. 20 



not been observed; the virgella is stout and 1.5 mm long. Thecae 

 12-14 in 10 mm; having the Climacograptus aspect in both views 

 in the proximal narrowed portion and throughout in the obverse 

 aspect, and the Diplograptus aspect in the reverse aspect; having 

 an average length of 1.5 mm and overlapping I Fig. 19 

 one-half of their length. The first thecae are 

 narrow and with their outer walls but slightly 

 inclined to the axis, the later thecae have oblique 

 straight ventral margins in the reverse aspect 

 and sigmoidal ventral margins in the obverse 

 aspect. The excavation in the latter aspect is 

 fairly deep (one-fourth the width of the rhab- 

 dosome) ; the apertural margin nearly straight. 



Horizon and localities. In the lowest part 

 of the Canajoharie shale at Morphy's and 

 Swartztown creeks near Amsterdam, N. Y., 

 and at the Carlsbad spring near Saratoga, N. Y. 



Remarks. This species is one of the various 

 forms that have been referred in our shales 

 first to Diplograptus -p r i s t i< s Hall, 

 and later to D. foliaceus Murchison ; but 

 it is readily distinguished from the rest by the 

 striking difference in the two aspects, the ob- 

 verse which is that of a Climacograptus, and 

 the reverse which is that of a Diplograptus. 

 In the Monograph of the British Graptolites 

 the subgenus Mesograptus is proposed for that 

 group of Diplograpti in which the stipe is con- 

 spicuously concavo-convex in section and the 

 appearance of the thecae different in the two 

 aspects. In the second of the two groups 

 again distinguished under the Mesograpti, the 

 excavations of the Climacograptus aspect are 

 deep and conspicuous throughout. This condi- 

 tion, for which the subgenus Amplexograptus 

 is proposed, is approached, if not reached, by pec t x 5. Carlsbad 

 the present form, although the excavations are Spring near Saratoga, 

 not as deep as in the typical Amplexograpti. N - Y. 



The species occurs almost to the exclusion of others and in pro- 

 fuse numbers in the basal beds at Swartztown creek and Morphy's 

 creek and is quite obviously characteristic of the lowest zone of the 



\ 



Fig. 19, 20. 



Diplograptus 



( M esograptus ) 



moha wken si s 



nov. 



Fig. 19. Reverse as- 

 pect, x 5. Swartz- 

 town creek, N. Y. 

 Fig. 20. Obverse as- 



