69G NEW YOKK STATE MUSEUM 



and Isograptus, and on tliis ground reunited this form Avitli Didyniograptus. 

 Lapworth, Elles and Wood Lave pointed out later [Joe. cit. p.oS] that the 

 observation made \>y Tciruquist, that " the first stipe crosses the sicula and 

 the second stipe the first tlieca," seems to indicate a deviation from the normal 

 Didymograptns type of development, that is to say, a forecast of the t}'pe 

 characteristic of the Diplograptidae, and that this, if substantiated, might 

 afford grounds for the letention of this form as the t}'pe of a subgenus. But 

 it seems to me that, Avithout entering into the details of the initial parts, D . 

 gibberulus differs in so important characters, that it clearly constitutes a 

 separate group, which, when the polyph}detic origin of the genus Didymo- 

 graptns shall be established, and the components of the various series be made 

 out, ^\■ill find recognition by a separate term. It represents the reclined group 

 of forms, in -which the branches grow straight upward, and the thecae are in 

 contact throuQ^hout their lena;th. 



Nicholson has asserted that tAvo very different forms had been thrown 

 together by Salter undei' the term D . c a d u c e u s. He separated the biraraous 

 component as D. gibberulus, at the same time pointing out that this 

 species can be readily distinguished from the other component, tlie siuular 

 preservation of Tetragraptus bigsbyi, where but two branches are 

 preserved, by the fact that in D. gibl)ernlus the rhabdosome is widest 

 in its proximal part, Avhile in Tetragraptus bigsbyi it is narro\vest 

 there [comp. pl,12, fig.6 and pl.fS, fig.Ci]. Nevertheless, the two forms have 

 probably been confused on more than one occasion, and D . c a d u c e u s may 

 for this reason have a still Avider distribution than is accorded to it. 



If Salter indeed comprised two forms under one specific term, according 

 to present usage, the form Avhich he figured first has still to retain his name, 

 afid the other to be separated under a new name. The two forms now, which 

 are thought to be united under Salter's term, viz D . g i b b e r u lu s Nicholson 

 and T e t r a g r a }) t u s bigsbyi Hall, bear indeed a superficial resemblance, 

 when the latter is so preserved that but two branches are seen. But, as 

 Nicholson, and later Elles have asserted, they can be readily distinguished by 

 the aspect of the proximal part. Using this criterion, the reliability of which 



