92 SYNOPTIC COLLECTION. 



is a front view showing especially the form and position 

 of the second and third thecae, and fig. 7 shows four thecae 

 and the partly imbedded sicula. In fig. 8 it is seen how 

 the thecae extend more and more over the sicula until the 

 latter becomes incorporated in the main mass or hydro- 

 zoina. At this time the distal end of the virgula begins 

 to grow, and it becomes stouter the farther it gets from 

 the point of the sicula. In fig. 9 the distal end only of a 

 hydrozoma is drawn, the proximal end with its imbedded 

 sicula not being represented. 



It is interesting to note that Ruedemann * has shown 

 that some species of Diplograptus occur in large compound 

 colonies consisting of many branches or stipes united in 

 the center as seen in PI. 104. These hydroids probably 

 consisted of nutritive zoons possessing tentacles for catch- 

 ing food and cavities for digesting it. Besides these there 

 were doubtless other zoons which were reproductive in 

 function. The latter in the more specialized forms may 

 have freed themselves from the colony and swum away 

 as independent organisms or medusae. That medubae 

 lived as far back as the lower Cambrian has been proved 

 by Walcott. 2 As we come down to the present time we 

 find the probable representatives of the Graptolites in 

 the Plumularian hydroids, Aglaophenia (No. 105) and 

 Sertularia (No. 106; no. 107, dried specimen). The 

 former has the thecae on one side of each branch, while 

 Sertularia has them on both sides. These hydroids have 

 reduced characters, since the reproductive buds or gono- 

 phores, which in a progressive form swim away as free 

 medusae, here never become detached. These are finely 

 shown in Sertularia argentea Ellis and Sol. (No. 106). 



J Rep. State Geol. N. Y., 1894, p. 219. 



2 U. S. Geol. Surv., Monograph, XXX, 1898. 



