Development and Mode of Growth of Diplogeaptus. 225 



sist of shale and are apparently the casts of large vesicles. In 

 the original of PL II, fig. 2, a smooth segment projecting from 

 the center of the colony is visible ; in the specimen represented 

 on PI. I, fig. 10, a plate is visible, which has a diameter of 6 mm., 

 and is raised in the middle to about 1 mm. Figure 7 (Pi. II) is 

 taken from a specimen in which the cyst is broken out, leaving 

 only its outline and a wide pit. The central organs are visible 

 at the bottom of the pit, while the rhabdosomes proceed from 

 the base of the little projection of sediment. It must be inferred 

 from this and other specimens that the vertical order of the 

 organs was as follows ; basal cyst, gonangia and rhabdosomes, 

 both of the latter proceeding from the central disk and enclosed 

 funicle. 



Whether the large vesicle' was the upper or undermost of the 

 organs is a problem of great interest on account of its bearing on 

 the. question of the function of that organ. The fact that most 

 specimens found on the surface of the shale, show it only as an 

 impression, while on the original of PI. I, fig. 10, which was 

 taken from the under side of the Graptolite-bearing layer, it is 

 preserved in relief, led the writer to the conclusion that it repre- 

 sents the topmost part of the colony. This conclusion and the 

 fact that the vesicle is often found filled- with sediment, and, 

 therefore, may have been hollow, have suggested the comparison 

 of the vesicle with a " float," such as certain Siphonophora^ viz., 

 the DisGoidece, p ssess. 



There are other observations which would seem to be in con- 

 cordance with the assumption that the colony of Dij^lograjytics 

 had a floating habit : 



1. The extreme length and thinness of the hydrocaulus in cer- 

 tain specimens of Dijplograptus In the State Museum at Albany, 

 N. Y., the writer has observed a rhabdosome of Diplograptus 

 with a length of 4 cm. and a breadth of 3 mm., while the hydro- 

 caulus has a breadth of only 0.1 mm. It is difficult to imagine 

 how such an extremely thin stem could have supported the long 

 and broad rhabdosome in any other than a suspended position.* 



* Carl Wlman (cf. Ueber die Graptollten, Bulletin of the Geol. Instlt. of Upsala, No 4. Vol. II, Part 

 2, 1895, p. 68) has pointed out that the vlrgula could have served only to strengthen the rhabdosome. 

 This rod extended also, as will be shown later. Into the hydrocaulus. An effort at strengthening the 

 latter, however, Is strongly suggestive of a sessile mode of life of the colony. 



29 



