56 
PALAEONTOLOGY OP NEW-YORK. 
438. 17. LINGULA ACUTIROSTRA*. 
Pl. XX. Fig. 5. 
Lingula acutirostra. Hall, Geol. Rep. 4th Dist. N. York, 1843, p. 77 ; fig. 9, p. 76. 
Shell broad below, tending abruptly to an acute point at the apex; surface marked with a 
single series of rather coarse striee. 
This species is larger than L. acuminata , and the surface more strongly marked by the con¬ 
centric strife, as well as being more abruptly acute in form. 
439. 21. ORTHIS CIRCULUS. 
Pl. XX. Fig. 6 a - g. 
Orthis circulus. Hall, Geol. Rep. 4th District, 1843, pag. 71, fig. 1. 
Shell nearly circular, wider than high, nearly equivalved ; surface finely radiated $ rays 
arcuate, dichotomous, curving upward and running out on the hinge-line; hinge area narrow 
and short, being little more than one-third the width of the shell; dorsal valve somewhat de- 
pi essed near the base, and the opposite valve with a corresponding elevation ; beak of dorsal 
valve small and neat, slightly curving over the area; beak of ventral valve scarcely rising 
above the area line. 
Fine concentric lines are barely visible in some parts of the shell, crossing the radiating 
striae. These lines are probably conspicuous in other specimens : in the one before us, the sur¬ 
face has been partially injured by the decomposition of iron pyrites. The depression noticed 
in the dorsal valve is broad and undefined, as likewise -the elevation on the corresponding 
valve. 
In many respects this shell resembles the 0. hybrida of the Niagara group, but differs from 
that species in its larger size, narrower area, and in the beak of the ventral valve being less 
elevated above the area line. In the interior, the figures of the casts of the two species show a 
decided difference. The 0. circulus is similar in size and form to, and would readily be mistaken 
for, the species of the Delthyris shaly limestone ; but in the latter the beaks are more nearly 
equal, the area almost equal on the two valves, the striae are less sharp and less arcuate, and 
the interior muscular impression and other characters differ still more widely. From the allied 
species of the Hamilton group, it differs still more widely than from either of those named. 
The several species of Orthis, of which 0. circulus is the earliest type known in our strata, 
approach each other in external characters so nearly that it requires careful observation to 
discriminate between them. They should not, however, on account of their similarity, be con¬ 
founded with each other, nor regarded as a single species, which gives rise to much confusion 
and difficulty in geological investigations. 
*The specimen from which the description was made, and the only one known to me, has been lost or mislaid 
since the Geological Report was published. The species L. elliptica, described in my Geological Report of 1843, I do 
not include here; having some suspicion that it does not belong to the Clinton group, and being unable to assign its 
true position. 
