448 JDr. 0. Serrmann — Distribution of Graptolites. 



semble the Black Dog rock, and these abound in the minute, thickly 

 aggregated fibrolite. There is a generally similar association of 

 minerals in the cordierite-gneiss of Galgenberg. I am greatly in- 

 debted to Professor Judd for the loan of additional slides of the 

 Black Dog rock, made from my specimens, and for a series of 

 slides of cordierite rock from the above-named localities, and from 

 Burgstadt (near Chemnitz), Penig, Orijesvi (Finland), and Sertudalen 

 (Sweden). 



About half-way between the Black Dog rock and the mouth of 

 the Don, — and so about half a league N. of the latter, sundry blocks 

 of a dark rock are strewn on the shore, some of considerable size. 

 One is apparently a dark gneiss, others are evidently an igneous rock. 

 I collected two varieties, one representing the two largest blocks 

 which lie close together, as though they had once formed a single 

 mass. It is a heavy dark green rock speckled with grains of a lighter 

 colour, weathering brown. The following is a description: — holo- 

 crystalline, structure intermediate between granitoid and ophitic, 

 consists mainly of labradorite, augite and some olivine. The labrado- 

 rite is well preserved, exhibiting the usual lamellar twinning and 

 very bright colours with the crossed Nichols ; the augite, or rather 

 diallage, for the pinacoidal cleavage is generally well developed at the 

 expense of the prismatic, is light brown-coloured in many of the 

 grains, and exhibits the " schillerization " to which Prof. Judd has 

 lately drawn attention ; the olivine (not abundant) has its cracks 

 bordered with a staining of opacite ; some parts ai'e converted into a 

 dull brownish-green serpentine, but others are very well preserved. 

 A few lai'ger granules of black iron oxide occur in the slide. 

 Another specimen of a finer-grained rock contains no olivine, but a 

 little brown mica ; the pyroxenic mineral also is partly diallage, 

 partly a mineral of the hypersthene group, probably allied to the 

 variety called amblystegite ; it is distinguished by a more marked 

 dichroism, altering from a pale greenish to a bluish hue, and 

 extinguishes parallel with the pinacoidal cleavage. The mode of 

 occurrence of the two minerals, which are not always easy to 

 distinguish, is very similar, and they appear to have consolidated as 

 nearly as possible simultaneously. There is also some apatite in the 

 slide, and as usual some grains of iron oxide. 



III. — On the Organization and Economy of the Gkaptolithid^. 

 By Dr. Otto Herrmann.^ 



IN the first place, let us obtain a clear idea of a Graptolite, and for 

 this purpose we will consider two members of the family 

 Dichograptidge, namely, Didymograptus vacillans, Tullb. (Fig. 2, 

 p. 452), and Dichograptus octobrachiatus, Hall (Fig. 1, p. 449). 



The entire polypidom (Hydrosoma, polypary. Frond, " polypier," 

 "polypariet ") proceeds from a simple, hollow cone (sicula, "Fuss," 

 "Haftorgan," Eadicle or initial point ex parte), which, in the com- 



1 The second chapter of Dr. Otto Herrmann's memoir on the Dichograptidse, 

 the first chapter of which appeared in this Magazine for September, 1885. 

 Translated from the Nyt Magazin for Naturvidenskaberne, vol. xxix. pp. 160-176. 



