DEVONIAN FOSSILS 



91 



ferently placed, 

 the Columbus. 



N. seminodosus is found in 



Gigantoceras is coiled and has smooth 

 whorls, without tubercles, which distinguish- 

 es it from Centroceras and Nassauoceras. 

 G_. inelegans occurs in the Columbus and 

 Delaware. 



Ryticeras (fig. 222) has a loosely coiled 

 shell whose whorls do not touch. One of our 

 species has a series of frill-like expansions 

 of the shell which are very striking. When 

 the shell is cut through in quarrying blocks, 

 the frills look like spines; one specimen of 

 this kind is in the outer wall of the State 

 House in Columbus, to the right of the front 

 door. R. cyclops is the commoner species of the Columbus limestone. 



Nephriticerina has a shell of few whorls, not touching as in Ryticeras, but the whorls ex- 

 pand very quickly. Comparison of the figures will distinguish it from Ryticeras. _N. metula 

 (fig. 223) occurs in the Columbus. 



ANNELIDA. Worm tubes are found sparingly in our Devonian rocks, 

 genus Spirorbis, described in Chapter 4 (see fig. 43). 



They belong to the 



OSTRACODA. These tiny arthropods are abundant in some beds of the Ohio Devonian 

 but they are not easy to identify, even under the microscope. Many species occur in the Silica, 

 Olentangy, and Plum Brook shales and in the bone beds of the Columbus limestone. See Stew- 

 art (1936), Stewart and Hendrix (1945), and Stewart (1950). 



TRILOBITV Heads and tails of trilobites are not too rare in our Devonian. Finding 

 a complete specimen, like Phacops rana milleri (frontispiece) is quite an event. Enrolled 

 specimens of this species with the tail touching the head are more frequent than those lying 

 flat. The Silica formation of northwestern Ohio has long been known as a good collecting 

 ground for trilobites and that is where the one figured in our frontispiece came from. The 

 following key will serve both for isolated heads and tails or complete specimens. 



Key to the Commoner Devonian Trilobites of Ohio 

 ( see p. 4 for use of keys ) 



1. a) Specimen with only the head preserved 2 



b) Specimen with only the tail preserved 7 



2. a) Head with spines 3 



b) Head without spines Phacops 



3. a) Head with a central spine on the neck Anchiopsis 



b) Head without a central spine on the neck 4 



4. a) Central part of head wider at front 5 



b) Central part of head wider at the back Proetus 



5. a) Central part of head divided into lobes . . . .„ 6 



b) Central part of head not divided into lobes Trypaulites 



