184 Systematic Paleontology — Middle Devonian 



crease toward the margins by bifurcation and intercalation, and are 

 crossed by concentric lines of growth giving a reticulated appearance to 

 the shell. The muscular areas are conspicuous, outside of which the 

 surface is papillose, vascular markings are sometimes visible near the 

 margin and in the dorsal valve there is a septum in the upper part of the 

 muscular area. (For description see Nicholson 1874, and Nettelroth 

 1889.) 



As is generally the case with the Maryland material most of the speci- 

 mens are impressions, but a few presei-^'e the shell. The latter have been 

 compared with typical specimens of this species and A. spinosa Hall from 

 the Hamilton shales of New York and tliey clearly belong to A. reticularis 

 rather than to A. spinosa. The plications on a part of the Maryland 

 specimens are somewhat coarser than on those from New York, but are 

 not as coarse as the costae of A. spinosa and do not show at all the nodose 

 or strongly reticulated surface of tliat species. Specimens were shown 

 Dr. J. M. Clarke, who regards them as Hamilton representatives of A. 

 reticularis, clearly distinct from A. spinosa. There is a marked and 

 constant difference in large mimbers of specimens representing these two 

 species from Eighteen Mile Creek in western New York and apparently 

 only A. reticularis is represented in the Maryland Collection. This seems 

 odd because Atrypa spinosa is the species which has been reported from 

 Maryland and Virginia. A figure of this species is given by Rogers 

 from the Hamilton of Pennsylvania;' while Hall stated that "In 

 collections from the Hamilton group near Cumberland (Md.) and the 

 adjacent parts of Virginia, there are many casts and exfoliated shells of 

 A. spinosa, but none of them with the finer costae, or that can be 

 referred to A. reticularis." " And in another place it is stated that " In 

 casts of this species [A. spinosa^ from the Hamilton group of Marjdand 

 and Virginia, we find the same characteristic features preserved as above 

 described [under the specific description of this species].'" This identi- 

 fication of Hall apparently has been followed by later observers, for 



' Geol. Penna., 1858, vol. ii, pt. ii, p. 828, flg. G71. 

 = Pal. N. Y., vol. Iv, 1867, p. 324. 

 •■> Ibid., p. 323. 



