1 88 NATURAL SCIENCE. September, 



taking Monograptus dubius, Suess, as an example (see especially 14). 

 Figs. I and 2 represent the sicula end, Fig. i from the sicula side, Fig. 2 

 from the opposite (or anti-sicula) side. The sicula, except as regards 

 the passage to the first theca, is bilaterally symmetrical, and consists 

 of an older, less pointed, and thin-walled initial portion (s) prolonged 

 into a hollow rod (virgula, v.), and one younger, larger, apertural 

 portion (5'), provided with lines of growth and a mouth-spine (sp). 



The sicula, having reached a certain size, produces, as is shown 

 by the lines of growth in the periderm, a new individual, the first 

 theca (fj), which lies alongside the sicula and grows in the opposite 

 direction. This first theca again gives rise to another theca {t<i,), and 

 so on. The thecae in the Graptoloidea are all of one kind, but their 

 shape varies in different groups and species, and has been used as 

 the basis for division into genera. The thecse may be somewhat 

 cylindrical or prismatic in all their length ; or sometimes they may be 

 contracted at the mouth like the neck of a bottle, widening again into 

 a broad aperture ; or the outer edge of the mouth may be prolonged 

 like a roof above the next theca and so on.^ 



A zygograptus I consider to be a Didymograptus-like form, in which 

 the one branch is wanting, so that it has a certain resemblance to 

 Monograptus. From this genus, however, it is clearly separated in time, 

 which, perhaps, is of greater consequence in the case of graptolites 

 than in that of any other fossils, since the graptolites have such a 

 limited vertical distribution. 



Dimorphogmptns, whose sicula-region is constructed like that of 

 Monograptus, may be considered as a transition-form between Diplo- 

 graptidas and Monograptidae. 



Among representatives of the Leptograptidae, no one has as yet 

 come across any material fit for the examination of the internal 

 structure. 



The Diplograptidffi were examined almost simultaneously by 

 Tornquist (11) and myself (13). As regards the actual structure 

 itself we are in accord, but we differ as to the explanation of it and 

 the terminology to be employed. Here I shall first describe the 

 structure of a species of Diplogvaptus and of Cliniacograpfns hcckersiamis, 



^Cf. Jaekel (8) and Giirich (9). 



Early Stages of Graptoloidea. 



Fig. I. — Mo7iograptus dubius, sicula end from sicula side. Fig. 2. — The same 

 from anti-sicula side. Fig. 3. — Diplogvaptus sp., sicula end from sicula side. Fig. 4. 

 — The same from anti-sicula side. Fig. 5. — A later stage from sicula side. Fig. 6. — 

 The same from anti-sicula side. Fig. 7. — CUmacogyaptus kuckersiawis, sicula side. 

 Fig. 8. — Lower portion of same, anti-sicula side. All greatly enlarged. 



s, upper part of sicula. s', lower part of sicula ; in most cases this is placed 

 opposite the sicula-mouth. sp, mouth-spine of sicula. v, virgula. t^, to, etc., first, 

 second, and following thecae. a, "connecting canal" of Tornquist, "bud" of Holm. 

 I, longitudinal septum. 



