ee W. J. Harris: 



bed8. Trigojiof/rnjitus, Dulymogroptus v-deflexus, sp. nov., and 

 varieties of Didymograptus caduceus, range through all the series. 

 Oncor/rajifvs iipsilon, T. S. Hall, and Dijilogropfiis, spp. indet., 

 characterise the lower beds, Cardior/rnpfus morsiis, gen. et sp. nov., 

 the middle beds, and Di plograpfus ijnonioiiioi.^, sp. nov., rano-es 

 througli the Middle and Upper beds. Sfrop/ioi/rapf iis fric/io/unnes, 

 Rued., also occurs. While it is inipractical)le in this part of the 

 paper to discuss details of structure, a few points which will be dealt 

 with more fully in the second part may be briefly indicated. 



(a) The close resemblance of the thecae of D. caduceus, Oiico- 

 graptus, and Cardingrapfus'^ indicating a probable line of develop- 

 ment. 



(1)) The progressive development in form of rhabdosome from D. 

 caduceus to the Dicranograptid structure of Oncograptvs and the 

 biserial (?) Cardiograptus. Diplograjifus gnomonicus may represent 

 further development in the direction of simplification, for, Avliile 

 I have provisionally included it in the genus Diplogrojitus, it8 

 affinities with that genus are doubtful (Plate I., figs. 5 and 6). It may 

 be of interest to note its close resemblance to a form figured by 

 Ruedemann^ a.s PhyllograjJtus anna, J. Hall. While these figures 

 are given as of a phylogenetic, or senile, form of Phyllograptus, 

 they differ from a typical specimen of that genus in (1) the presence 

 of only two stipes. (Unless tlie draAvings are misleading, tlie stipes 

 normally shown as dark ridges along the medial plane, are absent, 

 but a virgula is shown instead). (2) The more rapid alteration of the 

 angle of inclination of the thecae. This will be seen by contrasting 

 figs. 28 and 30 with others on the same plate. Except for the some- 

 what greater width the two figures bear a striking likeness to D. 

 gnomonicus, which is certainly not a Phyllograptus. Another case of 

 resemblance which may be more than a coincidence is presented by 

 juvenile forms of» Oncograpfus and GardiograptKS, which recall the 

 form described by Ruedemann* as D. caduceiis, Salter, var. iiaiius 

 (Plate I., fig. 9). Did the tendency, wliich in America ceased at 

 this mutation, continue further and lead to the development 

 of Oncograptus and Cardiograptus 1 It would appear not unlikely, 

 for the horizon on which these forms are foynd seems to be that of 

 JJiplograjJtns deidatus in North America. 



1 Since this paper was written Mr. R. A. Keble has intimated that after a close examination 

 of Cardiograptus he has recognised a third branch which was prolialily at right angles to those on 

 the plane of the laminae. Such a habit infers a phyllograptid structure which was hinted at by the 

 Ute Dr. Hall in a verbal communication to the writer. 



2 Grap. N.Y., I., pp. 715-716, plate 1,5, figs. 28 and 80. 

 Ibid., p. 698, fig. 90. 



3 Grap. N.Y., p. 698, fig. 90. 



