t 



.,% .. / 



TKENTON LIMESTONE; 133 



172. 1. Var. A. DELTHYRIS LYNX. 



Pl. XXXII. D. Figs. 1 a-u. Si. \-v. 



Terebratula lynx. Eichwald, 1830, Skizze von Podolie, p. 2U'.i. 

 Spirifir lynx. Von Buch, 1s37, Ueber Dclthyiis, p. U. 



— — Id. ISK), Mem. de la Soc. i;(5ol. de France, Vol. iv. p. 100. 



— — KicHWALD, l^li), Sil. System in Esthland, p. ll.'i. 



— — Shkppardi de Ciistclnau, 1S-J3, Terrains siluriens de 1 'Amiriciue du Nord, p. 12, t. 11, f. 1.5. 



— — MuRCHisoN and Veh.neuii,, ISIT), Gcol. and I'al. Russia and the Ural Mountains, Vol. ii. 



pag. 130, pi. 3, fii;s. 3 a, S, and 4 a, b. ■ 



Shell scarcely transverse, varying from seniicUiptical to subquadrate and globose, often 

 nearly as thick as long ; length and width as 5 to 9, or equal ; cardinal line often exceeding 

 the width of the shell, and usually extended into short acute ears, or rounded at the 

 extremities ; area common to both valves, as also the foramen ; ventral valve more gibbous 

 than the dorsal valve ; surface marked with strong angulatcd longitudinal plaits, about 

 three to four of which mark the sinus, and four to five the elevated mesial lobe ; transverse- 

 ly ornamented by concentric or flexuous elevated subiml)ricating lines, which are very 

 obvious on the lower half of the shell, becoming very distinct in front, and, in perfect 

 specimens, continuing nearly to tlie beak. Examined with a magnifier, the surface is seen 

 to be covered with very fine granulations, like those in some specimens of recent Tere- 

 BR.\TUL.a;. 



It is impossible to assign any definite form or proportions to a shell as variable as this 

 species. The cardinal line, in young shells, usually terminates in small acute ears extending 

 beyond the width of the shell ; while in other cases, and particularly in the older and more 

 gibbous specimens, the cardinal line is less than tjic width of the shell, and its extremities 

 are rounded. There are, somewhat rarely, exceptions to the above observation, where the 

 cardinal line, in old individuals, still extends into small acute ears. The number of plica- 

 tions is very variable also ; but there is usually a relation between the number of those on 

 the sinus and medial lobe, and those on either side. The greater or less number of these 

 plications, however, has no reference to the form of the shell. 



This species, like many others of the Brachiopod.^, is influenced by local circumstances, 

 and, in its wide geographical distribution, presents varieties of form, or types peculiar to 

 different localities, dependent apparently upon the condition of the ancient ocean bed. In 

 the dark carbonaceous limestones of Nev/-York, it is almost always small ; while in the 

 lighter colored calcareous mud or shale of the West, it reaches a much greater size, and 

 presents a greater variety of form. 



The eastern type of this species presents the following characters in its different stages 

 of growth and development : 



In the young shell, there first appears liiree plaits in the sinus of the dorsal valve, with 

 four on the corresponding medial lobe of the ventral valve ; these four becoming two aijout 

 one half or two thirds of the distance from the base to the beak. As the shell grows older, 



