. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 206 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY sion; the rhabdosome passes from pendent through horizontal to reclined and eventually scandent. This change in direction of growth is an intermittently expressed tendency which affects all graptolites ever since D. flabelliforme reversed the normal orientation of a Dictyonema rhabdosome to begin the story of graptoloid evolution. It proceeds side by side with a sequence of changes in the development of the proximal end of the rhab-. Fig. 4. Monograptus argenteus (Nicholson
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 206 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY sion; the rhabdosome passes from pendent through horizontal to reclined and eventually scandent. This change in direction of growth is an intermittently expressed tendency which affects all graptolites ever since D. flabelliforme reversed the normal orientation of a Dictyonema rhabdosome to begin the story of graptoloid evolution. It proceeds side by side with a sequence of changes in the development of the proximal end of the rhab-. Fig. 4. Monograptus argenteus (Nicholson) x2 showing change in thecal character from hooked (proximal end) to straight (distal end); growth- lines on enlargements xlO of selected thecae are partly conjectural, and are inserted to illustrate the form and development of the thecae. dosome which may be expressed as a progressive delay in the formation of the two buds from which any bilateral rhabdosome develops (didymograptid, leptograptid, dicellograptid or diplo- graptid). A temporarily stable condition is reached when the first four thecae of the rhabdosome alternate in origin and there are three crossing eanals (as is very usual in dicellograptids and. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Harvard University. Museum of Comparative Zoology. Cambridge, Mass. : The Museum
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