Species o/* Didjmograpsus. 351 



otliers by the possession of a median radicle and two lateral 

 spines, placed on the same side of the frond as the cellules. 

 D.Jiaccidus, Hall, has three smaller spines placed in a similar 

 manner on the same side of the frond as the cellules (fig. 8) ; 



Fiff. 8. 



«, base of D.Jlaccidus, Hall ; h, base of D. anceps, Nich., showing the in- 

 ternal radicle ; c, base of another example of Z). anceps, in which there 

 is no radicle ; d, base of I), divaricatus, Ilall, showing the radicle with 

 its two lateral spines. All enlarged. 



hut the central spine of these is not the radicle^ as is shown 

 by the occurrence of the true radicle on the opposite side of 

 the frond — this completely altering the whole relations of the 

 parts. These anti-radicular ornamental spines oi D . jlaccidus 

 have, however, been confounded by Mr. Carruthers with the 

 genuine radicle with its flanking spines in D. divaricatus. 

 As regards the form of the cellules D. divaricatus cannot be 

 distinguished from D. sextans^ Hall, and D. anceps^ Nich. 

 The former, however, of these is readily distinguished by its 

 general form, and the latter, as I shall immediately explain, 

 is separated by the fundamental structure of the frond. 



Didymogvapsus Moffatensis, Carr., and one of the specimens 

 included under D. elegans, Carr., are clearly identical with one 

 another ; and both (unless figured upside down) appear to be 

 referable to D. divaricatus^ Hall, which bears the date of 1855, 

 and has therefore the priority*. 



Loc. Kare in the anthracitic shales of Glenkiln Burn, in 

 Dumfriesshire (Upper Llandeilo). 



Didymograpsus anceps^ Nich. PI. VII. fig. 5, 5a^6 h. 



(Geol. Mag. vol. iv. p. 110, pi. 7. figs. 18-20.) 



Frond consisting of two stipes, diverging from an initial 

 point which may or may not be marked by the presence of a 



* It is quite possible thai Didymograpsus (Chdograpsus) Forcha?nme?-i, 

 Geinitz, is really identical with D. divaricatus, Hall, in which ease Gei- 

 nitz's name would have to be retained, as it was published in 1852. 

 Accepting, however, the accuracy of the figure given by Geinitz (Die 

 Grapt. pi. 5. figs. 28, 29), the base appears to be destitute of the radicle 

 and lateral spines so characteristic oi T). divaricatus. The other figures of 

 Geinitz (ibid. pi. 5. figs. 30, 31) are certainly referable to a different form, 

 probalily to D.Jlacvidns, Hall. 



