I4 THE AMERICAN NATURALIST. [Vou. XXXII. 
matrixeon the grounds that they are rarely found in a partly 
decomposed state. From the appearance of the colonies and 
their fragments, it is concluded that they sank slowly through 
quiet water to the bottom. As the carbonaceous matter in the 
rocks is a steady companion of the graptolites, it is supposed 
to have sunk together with the latter. The source of this car- 
bonaceous matter is found by Lapworth in seaweeds, which, 
like the living sargassum, were pseudo-planktonic; that is, were 
originally sessile, but, being torn off, continued to live and were 
carried by the currents into all seas. Just as the richest fauna 
of campanularians is still to-day found on the floating seaweeds 
of the Gulf Stream, so the graptolites are supposed to have 
flourished on the floating masses of palzozoic algæ. To 
strengthen this theory, Lapworth calls attention to the fact 
that it has been found that the rhabdosomes are either fastened 
to a central disk or have at least a nema. Both central disk 
and nema indicate that the graptolites were sessile. The 
Dendroidea alone, which are never so common as the other 
graptolites, fastened themselves to rocks and stones, and 
belonged to the “ benthos”’ (lived at the bottom). Originally, 
however, all graptolites were benthonic, and become only later 
pseudo-planktonic. This change in the habitat necessitated a 
change also in the direction of the theca, which is indicated 
in the course taken by the first theca of Diplograptus (Fig. 2), 
and by the direction of the thecz of Monograptus and of the 
branches of Didymograptus, Tetragraptus, and Phyllograptus. 
It cannot be denied that the peculiarities of the distribution 
of the graptolites and their structure are well explained by 
Giirich’s and Lapworth’s theories. It is highly probable that 
many graptolites were indeed pseudo-planktonic, while some 
may even have gone further and have become free-floating or 
planktonic, and others are known to have been sessile at the 
bottom. 
