Species of Didymograpsus. 35^3 



measured between the stipes on tlic side opposite to tlic radicle, 

 is from 340" to 355°, the radicle is not furnished with lateral 

 spines, and the width of the stipes is extremely uniform in 

 any given individual. In D. divaricatiis, on the other hand, 

 the " angle of divergence," measured in the same way, is from 

 90"^ to 130°, the radicle is invarial)ly flanked by two lateral 

 spines, and the stipes are considerably narrower at their com- 

 mencement than towards their distal extremities. These 

 points of difference should be sufficient to prevent in future 

 any confusion between two species which in reality belong to 

 two different sections of the Didymograjysi. 



Loc. Upper Llandeilo, Dobbs's Linn, near Moffat. 



Didymograpsus flaccidus, Hall, sp. PI. VII. figs. 6, 6 a, 6 h^ 6 c. 



GraptoUthm Jlaccidus, Hall (Grapt. Quebec Group, Suppl. p. 143, pi. 2. 



fig3. 17-19). 

 Didymograpsus Jlaccidus (Nicholson, Geol. Mag. vol. iv. p. 110). 

 DidyiuograpsHs eleyans, Carruthers (in part), Geol. Mag. vol. v. pi. 6. 



figs. 8 i, 8 c. 



" Frond consisting of two slender, linear, flexuous stipes, 

 which are widely divergent from a small, short, obtuse radicle " 

 (Hall). The stipes are about one fiftieth of an inch in breadth 

 at their commencement, but widen out till a width of one 

 twenty-fifth of an inch may be attained, and they not unfre- 

 quently reach a length of several inches without showing any 

 signs of a termination. The proper " angle of divergence " of 

 the stipes, as measured on the opposite side of the frond to the 

 radicle, is from 280° to 320°, whilst the " radicular angle " is 

 from 40° to 80°. The radicle varies in length from one 

 twenty-fourth of an inch up to one tenth, being sometimes 

 long and pointed, at other times short and obtuse, whilst it is 

 invariably situated on the inferior or concave margin of the 

 frond. The margin of the frond immediately opposite to the 

 radicle is adorned by three short and delicate processes or 

 spines — one directly opposed to the radicle, and one springing 

 from the first cellule on each side (fig. 8 a) . These spines are 

 simply ornamental appendages, so to speak, and have nothing 

 whatever to do with the true radicle, from which they must be 

 carefully distinguished. The cellules are on the opposite side 

 of the frond to the radicle, from twenty-five to thirty in the 

 space of an inch, averaging tw^enty-eight, narrow, their outer 

 margins straight or very slightly curved, inclined to the axis 

 at a very low angle (about 20°), their apices usually gently 

 rounded, and the cell-apertures running partially across the 

 body of the stipe. 



As to the complete identity of this beautiful species with 



