696 NEW YOUK STATE MUSEUM 



and Isograptus, aud on this ground i-eunited tbis form with Didymogi'aptus. 

 Lapworth, Elles and Wood have pointed out later [Joe. cif. p.."33] that the 

 observation made by Toruquist, that " the first stipe crosses the sicula and 

 the second stipe the first theca," seems to indicate a deviation fi-om the normal 

 Didymogi'aptus type of development, that is to say, a forecast of the type 

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

 aft'oi'd grounds foi- the letention of this fonn as the type of a subgenus. But 

 it seems to me that, without entering into the details of the initial parts, D. 

 gi bb e r u 1 u s differs in no important characters, that it clearly constitutes a 

 separate group, wliich, when the polyphyletic oi'igin of the genus Didymo- 

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

 out, will find recognition by a separate term. It represents the reclined group 

 of forms, in \vhich the bi'anches grow straight upward, aud the thecae are in 

 contact throughout their length. 



Nicholson has asserted that two very different forms had been thrown 

 together by Salter under the term D. caduceus. He separated the bii'amous 

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

 species can be readily distinguished from the other component, the similar 

 preservation of Tetragraptus bigsbyi, where but two branches are 

 preserved, by the fact that in D. gibberulus the rhabdosome is widest 

 in its proximal part, while in Tetragraptus bigsbyi it is narrowest 

 there [comp. pl.l2, fig.6 and pl.15, fig.(>]. Nevertheless, the two forms have 

 probably been confused on more than one occasion, and D . caduceus 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, 

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

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

 and Tetragraptus bigsbyi Hall, bear indeed a superficial resemblance, 

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

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

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



