MARYLAND GEOLOGICAL SURVEY 195 



disk itself composed of vertical rods or spicules, with their extremities 

 expanded and more or less flattened to form the two surfaces of the disk. 

 The shafts of the spicules are cylindrical, about 1 mm. or slightly more in 

 thickness, and separated from each other by interspaces about equal to 

 their own thickness. The arrangement of the terminations of the spicules 

 upon the surface of the disk is in curved, radiating lines, crossing after 

 the manner of the engine-turned ornamentation of a watch. The ex- 

 panded outer extremities of the spicules are rhomboidal in outline, leaving 

 narrow, linear interstices on each side between adjoining spicules. A 

 short distance above the flattened, rhomboidal extremity there are four 

 connecting processes, which join the spicule with each of the adjoining 

 ones. The inner extremities of the spicules are also expanded and joined 

 together to form the inner surface of the disk." Weller, 1903. 



Occurrence. CHAMBERSBURG LIMESTONE (Echinospherites bed). 

 Southern Pennsylvania and at Pinesburg Station, Maryland. Black 

 River group of Canada, New Jersey, Kentucky, and Arctic America, 



Collection. U. S. National Museum. 



PORIFERA 



CLASS SPONGIAE 



Genus RHABDARIA Billings 

 RHABDARIA FRAGILIS (Billings) 



Rhabdaria fragilis Billings, 1865, Pal. Foss., vol. i, Geol. Surv. Canada, p. 357. 

 Rhabdaria fragilis Rauff, 1894, Paleontographica, vol. xl, p. 245. 



Description. " Small cylindrical stems, with a rough exterior, and a 

 central perforation or canal. At first these were thought to be silicified 

 specimens of Stenopora fibrosa, but when others were procured showing 

 the central canal, it became evident that they could not be thus referred. 

 They have the form of crinoidal columns, but are not jointed. No 

 structure can be made out in thin slices under the miWRWp^?' Billings, 

 1865. 



