6 THE AMERICAN NATURALIST. [Vou. XXXII. 
tidæ and Dicranograptide have not yet been isolated). The 
morphology of the entire colonial stock, however, is, owing to 
the frequent occurrence of only detached rhabdosomes, still 
little known. Hall (65) first described stellate groups of Dicho- 
graptidz from Canada; Hermann (‘g5) such from Scandinavian 
rocks; Moberg ('93) published a description of a Monograptus 
with disk; and Gurley (96) figured a Climacograptus with a disk- 
like expansion of the virgula. Ruedemann (95, '97) discovered 
colonies of two species of Diplograptus in Utica shale which. 
have been deposited under such conditions of quietude as to 
retain not only all the chitinous appendages of the mature col- 
onies, but also the successive growth-stages of the compound 
colonies. 
It appéars from the material that the rhabdosomes of Diplo- 
graptus formed umbrella-shaped colonies, consisting of rhabdo- 
somes of various length, which radiate from a central organ, 
“a 
O 
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a 
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Fic. _9. — Diplograptus Soliaceus Murch (D. ristis Hall): complete colony (Ruedemann). 
Fic. 10.— The same: a, Gonangium filled with sicule; 4, sicula developing into a stipe 
(Ruedemann). 
the central disk (Fig. 9). The latter, in its turn, is connected 
with a larger organ, the basal cyst, that probably served to 
secure the stability of the colony in the ooze of its habitat. 
The central disk is surrounded by a cycle of vesicles which 
