Species 0/ Didymograpsus. 339 



the third section of the genus, this state of affairs is reversed — 

 tlie " angle of divergence" being now on the opposite side of 

 the frond to the celhiles, whilst it is the " radicular angle " 

 which is included between the celluliferous margins of the 

 stipes. 



The genus Didymograpsus is characteristically, and, as far 

 as is yet known, exclusively, confined to the Lower Silurian 

 period. Not only is this the case, but the genus is very de- 

 cidedly more richly represented in the inferior portion of the 

 Lower Silurian series than in its higher portions. The genus 

 attains its maximum in the Skiddaw and Quebec group 

 (Lowest Llandeilo), where it is represented by no less than 

 nine British species and an equal number of American forms, 

 of which, however, some appear to be nothing more than mere 

 varieties. In the Lower Llandeilo or Arenig group we have 

 two very characteristic British species {D. geminus^ His., and 

 D. patulus, Hall), both of which occur also in the Skiddaw 

 Slates. In the Upper Llandeilo rocks we have four British 

 species, with at least one additional American form {D. serra- 

 tulus, Hall), which only occurs in Britain in the Skiddaw 

 Slates. In the Caradoc or Bala series no Dldymogropsi occur, 

 except in Ireland, where two species are found in rocks of this 

 age (Baily). In the most richly graptolitiferous Caradoc beds 

 which occur throughout Britain, namely the mudstones of the 

 Coniston series of the north of England, no single Didymo- 

 grapsus has hitherto been detected. The same absence of 

 Didymograpsi appears to obtain in certain strata in Scotland 

 which overlie the Graptolitic shales of Moffat, and which have 

 been recently described by Mr. Lapworth of Galashiels as the 

 probable equivalent Oi he Coniston Mudstones, under the name 

 of the Gala beds. In America, however, several species of 

 Didymograpsus are known to occur in the Utica Slate and 

 Hudson-River group, two formations which are believed to be 

 of Bala age. 



In the last edition of ' Siluria,' Mr. Carruthers mentions ten 

 species of Didymograpsus as occurring in Britain ; but some 

 of these cannot be retained as valid species. In the following 

 communication I shall describe fourteen species of the genus, 

 with which I am acquainted as occurring in Britain. Several 

 of these, however, have been already described as fully as -the 

 extant materials will permit ; and of these I sliall merely give 

 a short diagnosis accompanied by an illustration, so as to en- 

 able them to be readily recognized. 



Didymogi'apsus pcdidusy Hall, sp. PI. VII. figs. 1, \ a. 

 GraptolUhus patiflu.i, Hall (<ii'apt. Queboc (h'oup, p. 71, pi. 1. fip-s. lO-lG). 



