Ford — Primordial Fossils from Rensselaer County, N. Y. 371 



new class of fossils to the Troy fauna, and gives us, moreover, 

 another example of a fossil species having an extensive geo- 

 graphical range. 



It occurs in the coarse red-and-yellow-weathering slates of 

 the Lower Potsdam group at Troy. 



During the past season I paid a large number of visits to the 

 first band of limestone met with in going eastward from Troj, 

 and which, in a former paper, I have characterized as limestone 

 band No. 1. (This Journal, Aug., 1873 ) As the result of this I 

 succeeded in obtaining several species of fossils not previously 

 known from this band, although known to occur in the other 

 limestone beds of the Troy Primordial, and along with these a 

 single head of a new and very pretty Trilobite of the genus Ali- 

 crodiscus. This I shall describe for the present as follows : Head, 

 broadly rounded in front, nearly semi-oval in outline, greatest 

 width at about the mid-length, sbghtly narrowed in passing 

 backward from this point to the angles. Glabella conical, 

 about two-thirds the length of the head, with two straight, 

 moderately deep furrows extending all across, dividing the gla- 

 bella in advance of the neck-furrow, into three parts of nearly 

 equal length. Neck-furrow extending all across and deeper 

 than the other glabellar furrows. The form of the neck seg- 

 ment cannot be clearly made out owing to the damaged condi- 

 tion of the specimen at this point. Dorsal furrows narrow, not 

 deep, dying out toward the front of the glabella. Cheeks promi- 

 nent, much swollen in the posterior third, without eyes or 

 sutures. Marginal rim well defined all around, widest in front, 

 with a conspicuously raised edge, inside of which there is a 

 nearly flat or feebly concave space, and so bent upward in front 

 as to give to the head on a side view a kind of slipper-like 

 appearance On either side of the head, just inside of the raised 

 marginal edge, there are three small tubercles situated within 

 the limits formed by a line drawn across the head through the 

 middle of the cheeks and another drawn parallel with it just in 

 advance of the front of the glabella. 



Greatest width of the head 1^ lines ; length along the median 

 line, including the neck-segment, the same. Differs from Micro- 

 discus (Agnosim) lohatus Hall (Pal. N. Y., vol. i, p. 258, pi. Ixvii, 

 figs. 5 a-f ), from the same locality, in its shorter and transversely 

 furrowed glabella, its tuberculated margin, and in its general 

 proportions. 



For this species I propose the name Microdiscus Meeki, in 

 honor of Mr. F. B. Meek, whose labors in the cause of science 

 have so vastly contributed to advance our knowledge of Amer- 

 ican Paleontology. 



Occurs in conglomerate limestone of the Lower Potsdam 

 group at Troy. 



Troy, N. Y., Jan. 15, 1876. 



