TRENTON LIMESTONE. 1383 
172. 1. Var. A. DELTHYRIS LYNX. 
Pu, XXXII. D. Figs. 1 a-u, & a-v. 
Terebratula lynx. ExcHwaxp, 1830, Skizze von Podolie, p. 202. 
Spirifer lynz. Von Bucu, 1837, Ueber Delthyris, p. 44. 
— — Ip. 1840, Mém. de la Soc. géol. de France, Vol. iv. p. 190. 
— = Ercuwa.p, 1840, Sil. System in Esthland, p. 143. 
—_ — Suepparpi de Castelnau, 1843, Terrains siluriens de 1’ Amérique du Nord, p. 42, t. 14, f. 15. 
— _ Murcuison and Verneuin, 1845, Geol. and Pal. Russia and the. Ural Mountains, Vol. ii. 
pag. 136, pl. 3, figs. 3 a, 6, and 4 a, b. 
Shell scarcely transverse, varying from semielliptical 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 angulated 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 subimbricating lines, which are very 
obvious on the lower half of the shell, becoming very distinct in front, and, in perfect 
specimens, continuing nearly to the beak. Examined with a magnifier, the surface is seen 
to be covered with very fine granulations, like those in some specimens of recent TERE- 
BRATULE. 
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 the 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 Bracutopopa, 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 New-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 three plaits in the sinus of the dorsal valve, with 
four on the corresponding medial lobe of the ventral valve ; these four becoming two about 
one half or two thirds of the distance from the base to the beak. As the shell grows older, 
