62 



HYDROZOA. GRAPTOLITOIDEA 



a thin wall (b), and the broader or apertural part with a 

 thick wall marked with lines of growth (a). The pointed 

 part was probably the covering of the embryo, and the 

 broad part a later growth. The apical end of the sicula 

 is prolonged as a thread forming the virgula. A spine- 

 like projection is sometimes found at the apertural end 



Fig. 18. Early stages of Monograptus and Diplograptus (after Wiman). 

 Enlarged. 



A. Sicula of Monograptus, from the Silurian of Gothland, a, thick- 

 walled part with lines of growth ; b, the earliest part with a thin 

 wall ; c, spine-like projection from the apertural end of the sicula. 



Monograptus (same locality) with three hydrothecae developed. 1, 2, 

 3, first, second, and third hydrothecffi ; a, b, sicula ; c, spine-like 

 projection ; d, virgula. 



Diplograptus from Bornholm, with four hydrothecffi developed. 1, 2, 

 3, 4, first, second, third, and fourth hydrothecae ; a, sicula. 



B 



C 



of the sicula (fig. 18 A, c), but has no connection with the 

 virgula. In the development of Didymograptus a bud 

 is formed on one side of the sicula and from this arise 



