STUDY OF THE GRAPTOLITES. 241 



known of that character, and regarded as the typical forms of Didymo- 

 GRAPTUS, are similar to G. hifidus and G. extenuatus of the Canadian Decade, 

 represented in Plate iii, fig. 13, of this memoir, and differing from figs. 8 

 and 9 of the same plate only in the lesser divergence of the stipes. 



At a later period, Mr. Salter proposed a further subdivision of the 

 graptolites under the name Tetragraptus, " a kind of double Didymo- 

 GRAPTUS," of which G. hjronoides is made the typical species ; and G. 

 quadribrachiatus is referred to the same genus. He also proposes 

 DiCHOGRAPTUS for those having the " fronds repeatedly dichotomous from 

 a short basal stipe into eight, sixteen, twenty-four, or more branches, 

 each with a single row of cells." " But the main character which dis- 

 tinguishes DiCHOGRAPTUS is the presence of a corneous plate* which 

 envelopes all the lower part of the branches, and which is not known 

 in any other genus of the gi'oup ; it has not indeed been seen in more 

 than two or three species of Dichograptus, but it may not in many cases 

 have been preserved."! 



These subdivisions may be of some value when the entire frond and all 

 its appendages are preserved, but unfortunately this is a rare condition ; 

 and when we have but fragments of the stipes or branches, there is no force 

 or value in the application of these terms : we are thus reduced to the 

 necessity of adopting the old term Graptolithus. Again, the value of 

 Didymograptus I conceive to be pretty well illustrated in the case of G. 

 caduceus, the original of which is cited from Lauzon, Canada. J After 

 studying the large collection of graptolites made by the Canadian Geo- 

 logical Survey, I am compelled to believe that the G. caduceus was founded 

 upon such forms as I have represented on Plate iii, figs. 18-21; for we 

 have no two-stiped species or forms of " Didymograptus " with a pedicle 

 or radicle so long as that represented in the figures of Mr. Salter, nor 

 any one so abruptly recurved ; and I regard the apparent radicle in the 

 two examples figured as simply one of the four stipes imbedded in the 

 shale, and exhibiting its non-celluliferous margin and a small portion of 

 its width, as I have shown in the figures cited. 



Other varieties of this form show only the two simple stipes, with a 

 slight process in the centre. We have therefore a " Tetragraptus " in a 

 condition undistinguishable from a " Didymograptus ;" and the same may 

 happen in G. hryonoides, whenever the quadripartite stipe is separated 



* First discovered in the Graptolites of the Quebec group at Point Levis. 



t Quarterly Journal of the Geological Society, Vol. xix, p. 136. 



J Graptolithus caduceus (Salter), Quarterly Journal of the Geological Society, Vol. ix. 



Cab. Nat. 31 



