524 NEW YORK STATE MUSEUM 



which is exactly alike in its structure and which hence can be considered as 

 equivalent to that of the graptolites here under discussion. I copy here 

 for comparison the successive stages of a tubularian hydroid, Eudendrium 

 ramosum, from Allman [1872, pt2, pl.l3, fig.12-16]. The similarity of 

 the embryos in the graptolites and this hydroid is increased by the fact 

 that the embryo of the latter produces a delicate chitinous sac.^ 



In figure 4 the planula has "become fixed by a disklike enlargement 

 of one extremity." 



In figure 5 " the disk of fixation has become more decidedly 

 differentiated, while a delicate chitinous pei-isarc has become excreted over 

 the whole surface of the embryo." 



In figure 6 " the disk has begun to be divided into radiating lobes, and 

 the hypostoma has become differentiated." 



In figure 7 " the hydranth is now distinctly differentiated from the 

 hydrocaulus, while the tentacles have begun to sprout round the hypostoma, 

 and within a delicate chitinous sac, which envelops the whole." 



In figure 8 "the hydranth has attained to nearly its ultimate form, 

 and has burst through the chitinous sac, which has hitherto confined it, 

 and the tentacles are now free to extend themselves in the surrounding 

 waters." 



There is no visible ground for the assumption that, inasmuch as the 

 embryo of the graptolite shows the same composition of disk, nemacaulus 

 and conical sheath, the formation of its embryonic sheath did not take place 

 by the same processes of differentiation of an originally saclike covering 

 into a disk and closed cone, and the gradual lengthening of the apical part of 

 the cone into a nemacaulus. 



After the bursting of the embryonic sac, the zooid began to grow and 

 formed the apertural part of the sicula, the increment producing the trans- 

 versal growth lines. 



'The presence of the chitinous covering of the embryo in the graptolites has 

 repeatedly been cited as an important difference from the hydroids [p.576], but, as this 

 instance shows, without propriety. 



