Maryland Geological Survey 395 



extremities, nearly equal to the greatest width of the shell ; area narrow, 

 shorter than the hinge. Surface marked by ten to twelve plications on each 

 valve, of which the two on the middle of the ventral valve are larger and 

 more elevated than the others, and separated by a wider and deeper de- 

 pression than between those on each side. The central plication on the 

 dorsal valve is larger than the others near the front, but usually dies out 

 before reaching the beak. Shell marked by strong imbricating concentric 

 lamellge of growth." Hall, 1857. 



Length 8 mm. ; width 8 mm. 



Compared with New York specimens, the single example observed has 

 the ventral valve more carinated and the plications are somewhat smaller 

 and greater in number; the latter in typical examples vary from six to 

 eight, while the Maryland specimen has eleven. However, one New York 

 individual has been seen with as many plications. 



OccH/Tence.— Helderberg Formation, New Scotland Member. 

 Cheriy Eun, West Virginia. 



Collection. — U. S. National Museum. 



Family SPIRIFERID/E 



Genus SPIRIFER Sowerby' 



This well-known genus occurs in both hemispheres, beginning at the 

 base of the Silurian and persisting throughout the succeeding horizons of 

 the Paleozoic. It is characterized by great variation in form and exterior 

 ornamentation, but the internal features are far more constant. At 

 various times authors have grouped the species chiefly according to ex- 

 ternal characters, but as yet no satisfactory arrangement exists based on 

 ontogenetic and phylogenetic studies. For such work the literature cannot 

 be relied upon, and as there are probably not less than 500 named species, 

 no single collection is adequate. The grouping here adopted is that of Hall 

 and Clarke published in 1893. 



"■ See Hall and Clarke, 1893, Nat. Hist. N. Y., Pal., vol. ill, pt. ii, pp. 1-40. 



