HYDROZOA. GRAPTOLITOIDEA. 41 



one side of the common canal a row of small cups or 

 tooth-like projections is given off, these are the hydrothecce 

 (fig. 8 b, A), and each one opens on the one hand into the 

 common canal and on the other to the exterior ; the latter 

 aperture, known as the mouth (m) of the hydrotheca, is 



a b c 



FIQ. 8. Morphology of Graptolites. a, Portion of Monograptus perso- 

 natus ; b, vertical section of the same ; c, Monograptus showing the 

 dorsal groove in which the sicula was placed ; d, transverse section 

 of Monograptus with dorsal groove and virgula ; e, Monograptus 

 colonus, Coniston Grits, with sicula; f, Didymograptus murchisoni, 

 Llandeilo Beds ; g, Diplograptus foliaceus, Llandeilo Beds, with the 

 position of the embedded sicula indicated ; h, sicula of Diplograptus, 

 the earliest stage ; i, the same with virgula and two buds developed. 

 (All enlarged except f.) 



a, axis or virgula ; c, common canal ; h, hydrotheca ; m, mouth of 

 hydrotheca ; s, sicula. 



