130 



COELENTERATA 



PHYLUM II 



uiiiserial rows of prismatic thecae coalescing along the whole length of their 

 dorsal margins ; the entire structure resembling Tetragrcqitus with the four 

 branches grown together, each two back to back and forming a cross in 

 transverse section. Ordovician. 



Family 2. Leptograptidae Lapworth. 



Uniserial, Jlexuous, bilateral rhabdosomes, with simple or compound lateral 



branches; thecae with a slightly sigmoid curvature, 

 apertures inclined, somewhat introverted. 



Leptograptus Lapworth. Rhabdosome consist- 

 ing of two long, filiform, bilaterally symmetrical 

 branches. Ordovician. 



Nemagraptus Emmons (Coenograptus Hall) (Fig. 

 203). Two primary branches originating from the 

 Fif; 2(13. centre of a triangular sicula, more or less flexed, 



Cocnmjraptus nrnriUs Hall. Ordo- and giving off simple branches from the convex 

 vician; Point Levis, Canada (after gide at approximately regular intervals. Ordovician. 



Family 3. Dicellograptidae Lapworth. 



Uniserial or uni-biserial Graptoloidea. Thecae tubxdar, with conspicuous 

 sigmoid ventral curvature. Apertures situated in excavations and frequently intro- 

 verted a,nd introtorted. 



ig 



Dicellograptus Hopk. Rhabdosome bilaterally symmetrical, consistin 

 two uniserial branches diverging from the sicula at angles ex- 

 ceeding 180 degrees. Ordovician. 



Dicranograptus Hall (Fig. 204). Rhabdosome Y-shaped, com- ^ || 

 posed of two symmetrically developed bi'anches which are 

 coalescent in the proximal and free in the distal portion of 



of 



their length. 



Ordovician. 



Fig. 204. 



Dicranograptus 

 ramn.sns Hall. 

 Ordovician 

 (Hud.son River); 

 New York (after 

 Hall). 



Suborder B. AXONOPHOEA Freeh (emend. Ruedemann). 

 Family 1. Diplograptidae Lapworth. 

 Biserial Graptolitoidea with rectilinear rhabdosomes. 



Climacograptus Hall (Fig. 205, a, c). Rhabdosome bilaterally 

 symmetrical. Thecae tubular, ventral walls sigmoidally curved, 

 apertural margin horizontal, situated within a well-defined ex- 

 cavation. Ordovician and Silurian. 



Diplograplus M'Coy (Figs. 205, d-f ; 206). Rhabdosome bilaterally sym- 

 metrical, rectangular, concavo-convex or tabular in section. Thecae mostly 

 sub-prismatic, ventral walls inclined and straight. Subgenera : Orthograptus 

 and Glyptograptus Lapworth ; Mesograptus Elles and Wood ; Petalograptus 

 Suess ; Cephalograpt'us Hopkinson. Ordovician and Silurian. 



Family 2. Glossograptidae Lapworth. 



Biserial Graptoloidea with straight rhabdosomes, test attenuated, with framework 

 of strengthening fibres. Thecae of Diplograptid type, provided ivith spurs and other 

 processes which often form an external lacework. 



Glossograptus Emmons (Figs. 207, 208). Rhabdosome having Ungulate 



