On the British Species o/* Did jmograpsus. 337 



XXXVI. — On the British Species of Didymograpsus. By 

 Henry Alleyne Nicpiolson,M.D.,D.Sc.,M.A.,F.R.S.E., 

 F.G.S., Lecturer on Natural History in the Extra- Acade- 

 mical School of Edinburgh. 



[Plate VII.J 



The genus Didymograpsus was originally proposed by M'Coy 

 (1851), to include those Graptolites which are " bifid from the 

 base" (Palasozoic Fossils, p. 9). In the year 1852, Geinitz 

 proposed the genus Cladograpsus, chiefly for such forms as had 

 been intended by M'Coy to be placed under Didymograpsus. 

 With these, however, he placed species which have been sub- 

 sequently removed by Hall to the genus Dicranograpsus (e.g. 

 D. ramosus). Still, under the head of Cladograpsus Geinitz 

 placed none but such forms as were understood by the spiecies 

 gemeUtc of Bronn, or, in his own words, " zweiarmige oder 

 gabelformige Graptolithinen." Recently the genus Clado- 

 grapsus has been redefined by Mr. Carruthers, and has been 

 made to include two generic forms which not only are in no 

 sense " species gemellffi," but which differ from one another 

 so widely that tliey cannot be placed under the same genus at 

 all (viz. Pleurograpsus linearis, Carr., sp., and Ilelicograpsus 

 gracilis, Hall, sp.). There can be no hesitation, however, in 

 retaining tlie term Cladograpsus simply in the sense in which 

 it Avas employed by its original inventor — namely, as a syno- 

 nym for Didymograpsus. 



The genus Didymograpsus was rejected by Hall upon very 

 insufficient evidence, in the belief that all the forms included 

 under this head would be found ultimately to be fragmentary, 

 and to be merely portions of compound Graptolites. Hall, 

 however, has failed to show that this is the case, in America, 

 with any other species than D. caduceus, Salt., which he 

 proved satisfactorily to be referable really to Tetragrapsus 

 hryonoides, a four-stiped species. No British palaeontologist, 

 however, doubts for a moment the integrity of the forms re- 

 ferable to Didymograpsus ; and, in point of fact, the genus is 

 one of the most natural in the whole family of the Grapto- 

 litidaj. 



The genus Didymograpsus may be defined as comprising 

 those Graptolites in which the frond is bilaterally symmetrical 

 and consists of two monoprionidian branches springing from 

 an " initial point," which is generally marked by a distinct 

 mucro or " radicle." In some cases the radicle may be very 

 rudimentary, as in D. sextans, Hall, and in some examples of 

 D. hifidus. Hall ; and it seems sometimes to be even altogether 

 absent, as in numy specimens of D. anceps, Nich. 



