STUDY OF THE GRAPTOLITES. 209 



SINGER, and G. murchisoni, Beck, which arc by no means nearly related 

 to G. ramosus or G. furcatus. The first named two species, which were 

 the earliest known of that character, and regarded as the typical forms of 

 DidymogTaptus, are similar to G. hifidiis and G. extenuatus of the Canadian 

 Decade, represented in fig. 13, Plate iii of this memoir, and diff"ering 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 Tetragraplus, " a kind of double Didyviograp- 

 tus,^^ of which G. hryonoides is made the typical species ; and G. quadri- 

 hrachiatus 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 distinguishes 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 group ; 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. "t 



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

 its appendages are preserved, but unfortunately this is rarely so ; 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 neces- 

 sity of adopting the old term Graptolithus. Again, the value of Didy- 

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

 ctiduceus, the original of which is cited from Lauzon, Canada. t 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 

 into two ; and in the separated stipes it is impossible to know if 

 there have been two, four, or eight in the entire individual. With 

 regard to those fronds which are repeatedly dichotomous, forming 

 the genus Dichograptus, of which the distinguishing character is the 

 central "corneous plate which envelopes all the lower part of the 



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



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



X Graptolithus caduceus, Salter; Quarterly Journal of the Geol. Society, Vol. ix. 



[Assein. No. 231).] 27 , 



