3H 



NEW YORK STATE MUSEUM 



a little more than one third their length ; the ventral wall of their free 

 portion strongly convex ; the apertural part introverted and introtorted, con- 

 tained within a semicircular excava- 

 tion which occupies one half the width 

 of the branch and two fifths the length 

 of the theca. 



Position and locality. Occurring" 



^^ 



^iX" /%SSr S^sjT\ in great profusion in the upper shaly 

 \VI Trenton limestone near Pratt's Ferry, 



^s^v Bibb co., Ala. 



Remarks. The form of the rhab- 

 dosome in the compressed condition is 

 varying between wide extremes, which 

 can be but partly due to different direc- 

 tions of compression. While the vast 

 majority of the specimens have the 

 form above described and considered 

 here as the typical expression of the 

 species, a few have been observed to 

 diverge directly at an angle of 270° 

 [see fig. 4] and to approach more or 

 less closely to D. sextans in their 

 Fig. 237, 238 Diceiiograptus smithi sp.nov. Fig. form, while in others the branches 



237 Group of typical specimens. Fig. 238 Portion of 



branch showing thecae distinctly. x 5 cross already closely to the sicular 



extremity. This variability can only be taken to mean that the species 

 is in rapid development or transition to another type. Since the form 

 of some of the specimens, as well as the character of the thecae, evince 

 close relationship to D. sextans, while others in the approach ot 

 the bases of the branches, which is seen in all stages of evolution, are 

 clearly well on the path to a Dicranograptus, it can be properly sur- 

 mised that we have here before us a very interesting transitional form from 

 a Diceiiograptus of the type of D. sextans to a Dicranograptus ot the 



