Development and Mode of Growth of Diplograptus. 221 



found in countless multitudes in shales on both hemispheres only 

 Slppeared in single stipes. The specimens which I have found 

 show that this genus also grows in compound fronds. The 

 extension of the axis at the growing end of Diplograptus has 

 presented unsurmountable difficulties to the ett'orts of explana- 

 tion. The corneous cup which was observed by Nicholson* on 

 Climacograptus hicornis, Hall, the vesicular dilatations of 

 Diplograptus p/iysophora, Kich., and of Diplograptus pristis, 

 Hali, are at the sicular end and are, therefore, other organs 

 than the central disc from which the stipes branch. Neither can 

 the prolonged vesicle in the antisicular prolongation of the axis 

 of Diplograptus vesiculosus, Nich., be compared with the central 

 disc. 



General Form of the Frond. 



Typical views of the complete fronds are given in PL I, fig. 1, 

 D.pristis, Hall, and in PI. I, fig. 2, of D. Euedemanni, Gurley. 

 As figure 1 shows, there are in a frond stipes of very different 

 lengths. In this specimen, in which some of the stipes, seem 

 from their dimensions, to have attained their full growth, we 

 notice stipes of three different lengths. Four stipes, lying in two 

 diameters, perpendicular to each other, are the longest. They 

 are accompanied on each side by shorter ones. Stipes of about 

 the same length as the latter bisect the right angles. Between 

 the others, we find the shortest stipes in varying number. The 

 origiral specimen for figure 1 has 26 stipes; but fronds with as 

 many as 40 stipes have been found, in which most of them have 

 reached the normal length. Yery often we find fronds with 

 only one or a few stipes of the first, and numerous stipes 

 of the third length. It is probable that these very different 

 lengths of the stipes in the frond indicate different age and not 

 that it grew out as a whole, thus maintaining always the same 

 proportions in the stipes, as Dr. O. Herrmannf asserts in regard 

 to the frond of the compound Monoprionidae. 



The number of stipes of D. pristis, Hall, is considerably 

 greater than that of D. liuedemanni, Gurley ; the former 

 showing between 20 and 40, the latter only about 12 stipes. 

 The fronds of D. pristis, Hall, therefore, are usually crowded 



♦ Ann. and Mag. of Nat. Hist., 1868, vol. I, p. 55. 



t On the GraptoUtlc Famllj' Dichograptidce Lapw., The Geol. Magazine, 1886, p. 13. 



