i-si^ 



6 



m. 

 hth. 



Fig. 123. 



B 



Fig. 124. 



Fig. 123. A, Part of a colony of Monograptus. B, Diagrammatic vertical 

 section of same. C, Monograptus colonus with sicula {si.), ex. common canal; 

 hth. hydrotheca; m. mouth of former; vr. virgula. From Woods. 

 Fig. 124. A, Didymograptus v-fractus. Ordovician. Early part of the poly- 

 pary. After Elles. si. sicula ; cr.c. crossing-canal ; i, first hydrotheca ; 2, second 

 hydrotheca. x 5. B, Tetragraptus similis. Lower Ordovician. Young form 

 with virgula and disc. After Ruedemann. x 4. 



Fig. 126. 

 Fig. 125. Tetragraptus. Ordovician Rocks, a, central disc, x \. 

 Fig. 126. Diplograptus foliaceus from the Utica Slate, New York, x 

 After Ruedemann. For description see text. 



