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NEW YORK STATE MUSEUM 



in Tetragraptus fruticosus [see Mem. 7, text fig. ii, also Ibid. 

 pi. 10]. In those groups of Tetragraptus and Diclymograptus, in which the 

 recurving of the branches takes place close to the sicula, as in Tetra- 

 graptus similis [ibid, text fig. 58] the sicula and first thecae become 

 spinose. Frequently the difference in aspect between these first, strongly 

 spinose thecae and the later ones is strongly marked [L ep tograp t u s 

 flaccidus var. spin if er]. In several species of Didymograptus (D . 

 sub t e n u i s) and Nemagraptus (N. exilis Lap worth, text figure 4) 

 only the sicula and one theca on either side are armed with spines, but 

 these are found exactly where the change in direction of the branches takes 



Fig. 4 Nemagraptus exilis var. linearis nov. Shows spines of primary thecae. x 5 Fig. 5 

 Diplograptus angustifolins Hall. Sicular end showing character of spines, x 7 Fig. 6 GI o sso- 

 graptus quadriniucronatus var. corn ut us nov. Sicular end showing lateral spines, x 7 Fig. 7 

 Cryptograptus tricornis (Carruthers). Aperture of sicula showing four spines, xj 



place and thereby a part of the branch is projected. Among the Diplo- 

 grapti either the virgella alone grows out needlelike, or it is accompanied 

 by the opposite spine of the sicula and two more powerful spines grow from 

 the first two thecae ; this even in forms where the later thecae are wholly 

 unarmed, as in Diplograptus angustifolins [see text fig. 5] and 

 D. foliaceus [see text fig. 286]. In some varieties, as in D. folia 

 c e u s var. calcaratus and D . trifldus Gurley these spurs attain such 

 relatively great size that they become the most striking feature ol the 

 colony [see text fig. 295 ]. In Gloss g r a p t u s qua d r i m ucro n atus 

 the thecal apertures arc each protected by a pair of lateral spines, but the 



