STROPHOMENA. 77 



Strophomena depressa : Vanuxem, (Jeol. Report Third District New-Tork, 1842, pa. 79, f. 5. 



S. undulata .- Id. lb., p. 139, f. 3. 



S. depressa .■ Hai.l, Geol. Report Fourth District New- York, 1843, p. 77, f. 5; and p. 104, f. 2. 



Orthis rugosa : F. C. RcEMER. Rhcin. Uebergangsgebirge, 1844, pp. 85 & 90. 



Leptxtia depressa : De Vekneuil, Geol. Russ. aad the Ural, 1845, Vol. ii, pa. 234, pi. 15, f. 7. 



Leptana tenuUtriata : Haix, Palseontologj' of New- York, Vol. i, p, 108. 



Leptcena depressa .- Hall, Palajontology of New-York, 1852, Vol. ii, p. 257. 



Strophomena rhomboidalis : David.son, Intr. Nat. Hist. Brachiopoda. 



" " : Ib., Monograph of the British Carboniferous Brachiopoda. p. 119. 1860. 



Strophomena rugosa' : Hall, Palajontology of New- York, Vol. iii, p. 195. 1859. 



The shell is more or less semielliptical or subquadrate, varying greatly 

 in its proportions of length and breadth : hinge-line straight ; car- 

 dinal extremities sometimes rounded, sometimes acute and slightly 

 produced. The valves are geniculated, and the proportions of the flat- 

 tened part or disc and the recurved part of the shell are very variable, 

 insomuch that the geniculation is sometimes little more than one of the 

 strong concentric wrinkles. 



The surface of the flattened portion is marked by concentric (and 

 sometimes interrupted ) wrinkles, which, following the curve of the 

 outline, are bent outwards, and often become obsolete on the cardinal 

 angles. These concentric wrinkles are very variable in number, being 

 from six to fifteen or sixteen upon specimens from the same rock. The 

 entire surfece is covered by radiating threadlike striae. 



In young specimens there is usually a round foramen in the apex of 

 .the dorsal valve, which becomes closed at a later period. The triangular 

 foramen of the ventral area is partially closed by a deltidium and the 

 apex of the ventral valve. 



This species is not of frequent occurrence in the Schoharie grit, but is found 

 everywhere in the Corniferous limestone. The principal varieties are illustrated in 

 figures 16, 17 & 18 of Plate xii, the two latter being essentially casts of the 

 interior. In all the collections made in New- York, we have no specimens of this 

 species from the Hamilton or Chemung groups. 



• I adopted the name S. rugosa in the third volume of tlie Palaeontology, recognizing it as the ty])ical 

 species of the genus, and' thus named by Rafixesqiie. The name of Wahlenbero has precedence in 

 time, and will include those lower silurian forms described as Strophomena or Leptcena teit^utriata. 



