162 I Senate 



Spirifer tenuis ( n. s.)- 



Pal. N.Y. Vol. iv. 



Shell very thin and fragile, transversely oval, two-thirds to three- 

 fourths as lo ag as broad ; hinge about equalling the greatest 

 width of the shell, obtusely angular at the extremities : dorsal 

 valve convex; mesial fold broad, rounded ( 1 ),and marked the 

 whole length by a strong sulcus : ventral valve the more con- 

 vex, most prominent near the umbo; mesial sinus broad, giving 

 a sinuous outline to the anterior border, undefined at the mar- 

 gins, and having along the middle a deep groove, and on each 

 side several indistinct folds ; beak unknown ; area high, lon- 

 gitudinally and transversely striate, arcuate, and extending 

 obliquely beyond the hinge line ; foramen moderately large ; 

 lateral slopes of the valves ornamented by about eighteen or 

 nineteen very obtuse simple plications on each side of the mesial 

 sinus and fold : those on the dorsal valve more distinct than 

 those on the ventral, which do not reach the margin of the shell. 

 Surface marked by fine nearly obsolete radiating striae, which 

 are raised at intervals into granulations or papillse : these are 

 crossed by stronger, closely arranged, irregular, concentric lines 

 of growth. 



This species, in general form, agrees with the last ( S. arata) ; but is 

 less rotund, and the shell less elevated by the sinus in front. The longitudi- 

 nal furrow along the centre of the mesial fold corresponds to that species ; 

 but the plications are more rounded with broader spaces between them, and 

 they are scarcely conspicuous on the ventral margin of the dorsal valve, 

 and on the ventral valve are scarcely distinct below the middle of the shell. 

 The sinus is undefined at the margins, and the concentric linos very closely 

 arranged, giving the surface rather the aspect of Spirigera or Merista 

 than of Spirifer proper. 



Geological position and locality. In the shales of the Hamilton group : 

 Cumberland, Maryland. 



