194 SYSTEMATIC PALEONTOLOGY 



covering of hexagonal, cuplike plates fused or articulated by their edges. 

 On the exterior each plate is deeply concave and marked off at the surface 

 by a sharp wall. The plates are smallest at the narrow end of the organ- 

 ism, but increase in size in the more swollen part where an average 

 diameter for them of one millimeter is the rule. The interior of the 

 organism is hollow (pi. XLVI, fig. 5) and is frequently filled with crystal- 

 line calcite (pi. XLVI, fig. 3) . Specimens about 35 mm. in length are the 

 rule, but some individuals attain a length of 50 mm. 



Small fragments of Nidulites, especially where partly imbedded in the 

 rock, much resemble a massive bryozoan with large zooecia, but examina- 

 tion with a lens reveals the very different nature of the hexagonal plates 

 or cups. 



The systematic position of Nidulites is still quite uncertain. These 



bodies were supposed by Salter to be the egg-ribbons of marine gastropods. 



Later paleontologists placed them among the Protozoa, but to-day the 



prevalent opinion is that they are allied to Receptaculites, Cerionites, 



Ischadites and related genera, and are either sponges or calcareous algae. 



Occurrence. CHAMBERSBURG LIMESTONE (Xidulites bed). Pennsyl- 

 vania, Maryland, and Virginia. Wilson, Pinesburg Station, and other 

 localities in Maryland furnish numerous specimens of the species, em- 

 bedded in the rather massive strata of the Xidulites bed. 



Collection. Maryland Geological Survey, U. S. National Museum. 



Genus REGEPTACULITES Def ranee 

 RECEPTACULITES OCCIDENTALS Salter 



Plate XLV, Fig. 7 . 



Receptaculites occidentalis Salter, 1859, Canadian Org. Remains, dec. i, 



p. 45, pi. x, figs. 1-7. 

 Receptaculites occidentalis Billings, 1865, Pal. Foss., vol. i, Geol. Surv. 



Canada, p. 381, text figs. 354-356. 

 Receptaculites occidentalis Weller, 1903, Geol. Surv. New Jersey, Pal., 



vol. iii, p, 135, pi. vi, figs. 2-4. 



Description^- ^Sponge forming discoid or flattened, saucer-like ex- 

 pansions ; attaining a diameter of 200 mm., and having a thickness vary- 

 ing from 4 mm. at the center of the disk to 12 mm. at the margin. The 



