130 COELENTERATA phylum ii 



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

 dorsal margins ; the entire structure resembling Tetragraptus with the four 

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

 transverse section. Ordovician. 



Family 2. Leptograptidae Lapworth. 



Uniserial, flexuous, 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 



Fir.. 203. centre of a triangulär sicula, more or less flexed, 



Coenograptus iiraciiis Hall. Orrio- and giving off simplc branches from the convex 



cian- ""•■"'' ^"■"- ''" '■■ '■'""•- CO 



Hall). 



vician : Point Levis, Ciinada (after •!. '.i i-^ ^ r\ ^ ' • 



sideatapproximately regulär intervals. Ordovician 



Family 3. Dicellograptidae 'Lapworth. 



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

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

 verted and introtorted. 



Dicellograptus Hopk. Rhabdosome bilaterally symmetrical, consisting of 

 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 branches which are 

 coalescent in the proximal and free in the distal portion of 

 their length. Ordovician. 



Suborder B. AXONOPHORA Frech (emend. Ruedemann). 

 Family 1. Diplograptidae Lapworth. 

 Biserial Graptolitoidea with rectilinear rhabdosomes. pjo 204. 



Climacograptus Hall (Fig. 205, a, c). Rhabdosome bilaterally ramZT^Tl^i. 

 symmetrical. Thecae tubulär, ventral walls sigmoidally curved, ^Hudson mip^v 

 apertural margin horizontal, situated within a well-defined ex- New York (after 

 cavation. Ordovician and Silurian. 



Diplograptus 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 ; Cephalograptus Hopkinson. Ordovician and Silurian. 



Family 2. Glossograptidae Lapworth. 



Biserial Graptoloidea with straight rhabdosomes, fest attenuated, with framework 

 of strengthening ßbres. Thecae of Diplograptid type, provided with spurs and other 

 processes which often form an external lacework. 



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



