56 



rALAONTOLOor o» mew-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 scries of rather coarse striae. 



This species is larger than L. acuminata, and the surface more strongly marked by the con- 

 centric strise, as well as being more abruptly acute in form. 



439. 21. ORTHIS CIRCULUS. 



PL. XX. Fig. Co- 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, dlchotomous, 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- 

 piessed 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 imdefined, 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 O. 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 strise 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. 



*Thc specimen from which the description was made, and the only one known to me, has been lost or mislaid 

 since the Geological Keport was published. The species L. tllipticn, described in my Geological Report of 1813, 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. 



