Handbook of Paleontology 259 



the graptolites, the cystoids among the echinoderms, and 

 the straight-shelled cephalopods. 



Plants above the grade of seaweeds have not been 

 found in the Ordovician. Receptaculites and Solenopora, 

 forms very characteristic of this period and usually 

 placed with the sponges or corals, have recently been 

 shown to be plants. The former built hollow, dish or 

 vase-shaped skeletons, sometimes a foot in diameter, with 

 an inner and outer layer connected by pillars, and is easily 

 recognized even in fragments. Certain beds of the Ordo- 

 vician (Upper Chazy) are largly composed of them. The 

 Solenopora forms pebblelike masses consisting of very 

 small tubes arranged in radial manner from a point on 

 their base. The fact that Foraminifera and Radiolaria 

 are found in a few regions in great numbers indicates 

 that they were abundant in the Ordovician seas. Sponges 

 are represented by forms with silicious and calcareous 

 skeletons and, though rare in New York formations, are 

 very common in the Lower Ordovician limestones of the 

 southern Appalachian area. Graptolites here as in the 

 Canadian are important horizon markers, but there is a 

 completely different series. Among the characteristic 

 genera are Nematograptus, Climacograptus, Dicellograptus, 

 Dicranograptus and Diplograptus. The hydro corallines, 

 allied to the corals and represented by Stromatopora-like 

 forms which were abundant as reef builders in the Silu- 

 rian and Devonian, are found also in the Ordovician. 

 Corals were still rare but represented by several types, 

 among them the simple cup-corals, as Streptelasma. The 

 common forms were those living in colonies, as the large- 

 tubed Columnaria and peculiar forms (Tetradium) with 

 quadrangular tubes. The echinoderms were represented by 

 all groups, cystoids, Mastoids, crinoids, starfishes, brittle 



