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NEW YORK STATE MUSEUM 



about the same age (zone of Pleurograptus linearis). 

 Earlier mutations appear in New York in beds of Trenton age 

 (Canajoharie and Schenectady beds). One of these is characterized 

 by two long spines at the sicular end (mut. cornutus). 



In Glossograptus quadrimucronatus all four 

 sides or edges of the prismatic rhabdosome are furnished with a row 

 of spines each, two spines flanking each aperture. These spines are 

 of nearly equal length throughout and were obviously a protective 

 measure. 



The Utica shale at Holland Patent and South Trenton has afforded 

 a peculiar mutation that is characterized by spines of double length 

 (2 mm-j-) that are found opposite the sixth or seventh pair of 

 thecae, sometimes also opposite the fifth to seventh pair. In this 

 peculiar character the form exactly corresponds to the var. s p i n i- 

 g e r u s Lapworth of Glossograptus quadrimu- 

 cronatus (see Elles & Wood, p. 223), which singularly enough 

 occurs at the same horizon, that with Pleurograptus 

 linearis in the Hartfell shales of Scotland. In this vicarious 

 form of Scotland, however, the longer spines appear at about the 

 tenth pair of thecae. A third form has been described by T. S. Hail 

 (1906, p. 277), from Victoria, Australia. This has four prominent 

 spines about 2.5 mm long at about the seventh or eighth thecae. 



r^. 



Figs. 1-3 Glossograptus quadrimucronatus (Hall). Fig. i: 

 Mutation from the Utica shale at Holland Patent, N. Y. ; fig 2 : The variety 

 spinigerus Lapworth (from Elles and Wood); fig. 3: Mutation from 

 Victoria, Australia (from T. S. Hall). 



A fourth mutation has been observed in the Utica shale of Herki- 

 mer county, in an earlier horizon than is exposed at Holland Patent 

 In this form, in which the rhabdosome is abnormally large, the longer 



