On the British Sjyecies o/* Didymograpsus. 337 



XXXVI. — On the Jin'ti'.sh Sj)€cies of Didyinograpsus. By 

 Henry Alleyne Nicholson, M.1).,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 Di<hjmograpsns was originally proposed by M'Coy 

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

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

 proposed the genus CJadogra^isiis^ chiefly for such forms as had 

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

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

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

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

 placed none but such forms as were understood by the species 

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

 gabelfbniiige 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 gemellaj," but which differ from one another 

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

 all (viz. Pleurograpsiis linearis^ Carr., sp., and Ilelicograpsus 

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

 retaining the term Cladograpsus simply in the sense in which 

 it was emj)loyed by its original inventor — namely, as a syno- 

 nym for Didijmograp)SUS. 



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 

 hryonoide^, a four-stiped species. No British paleontologist, 

 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- 

 litidje. 



The genus Didymograpsus may be defined as comprisino- 

 those Graptolites in which the frond is bilaterally symmetriciu 

 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. sej:tanSj Hall, and in some examples of 

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

 absent, as in many specimens of D. anceps^ Nich. 



