240 Dr. H. A. Nicholson on new Species of GraptoUtes'. 



]iot turned clown at all. The common canal is extremely 

 narrow ; and the cellules form with it an angle of about 35°. 



This beautiful species joresents a superficial resemblance to 

 G. priodon^ Bronn. It is distinguished, however, by the slen- 

 der, linear, sharply curved base, the cellules of which resemble 

 those of G. Nilssoni^ by the much greater number of cellules to 

 the inch (those of G.priodon not exceeding twenty -five, whilst 

 those of G. argenteus are from forty to forty-five), by the great 

 comparative length and nearly uniform width of the cellules, 

 and by the great rapidity with which the stipe attains its full 

 dimensions. Further, it is only in the lower portion of the 

 adult part of the stipe that the cellules resemble those of G. 

 priodon^ since it is only here that they are reflexed at their 

 extremities. 



Loc. Abundant, and beautifully preserved in relief, in a 

 single thin band in the mudstones of the Coniston series, 

 Skelgill Beck, near Ambleside. 



Didymograpsus affinis^ Nich. PI. XI. fig. 20. 



Spec. char. Frond consisting of two extremely slender 

 stipes, each from half to three-quarters of an inch in length, 

 diverging from an initial point, which is provided Avith a long 

 and pointed radicle of about one line in length. The stipes 

 are very narrow, having a uniform width of from one-fiftieth 

 to one-fortieth of an inch. The cellules are on the opposite 

 side of the frond to the radicle; and the " angle of divergence"* 

 varies from 90° to 150°, the stipes being straight or gently 



* In the specific deterniinatiou of any Uuhimograpsus, one of tlie most 

 important points in tlie diagnosis is found in the " angle of divergence " 

 of tlie two stipes -wliich compose the frond. It is obvious, however, that 

 the two stipes, diverging from a single point, form two angles ; and it is 

 equally obvious that this character is absolutely valueless unless the same 

 angle be always chosen for purposes of comparison. The ordinary prac- 

 tice, in the case of the Didj/mof/rapsi, has been to take the sinaUcst angle, 

 and to call that the "angle of divergence," or, in cases where the two 

 angles were equal, to take the angle formed by the two stipes on the side 

 on which the cellules were placed. Now, in neither of these cases is it 

 really the same angle which is compared in diil'erent species ; or, at any 

 rate, it is not necessarily or invariably the same. The proper method of 

 comparison is to take the angle formed by the stipes on the opposite side 

 of the frond to the radicle, and to consider that as the 'Wangle of diver- 

 gence." In this way a constant standard of comparison is obtained, since 

 the radicle always marks the organic base of the frond. A neglect of this 

 obvious point has led to extraordinary confusion amongst the British 

 members of this genus, as I shall point out upon some future occasion. 

 The angle I'ormed by the stipes i/po/t the same side as the radicle may con- 

 veniently be called tlie ''radicular angle;" and the cellules, in dill'ereut 

 sections of the DidjpmH/rapsi, are found to occupy tho sides of the angle 

 of diverii-ence or of the radicular anii-le. 



