448 
PALAEONTOLOGY OF NEW YORE. 
Goniatites plebeiformis, n. sp. 
PLATES AVI, FIGS. 25, 26; CX, FIGS. 3-9. 
Porcellia? rotatoria. Hall. Illustrations of Devonian Fossils: Gasteropoda, pi. 16, figs. 25, 26. 1S76. 
Compare Goniatites phheiup, Barrande. 1S65. Syst. Silur. du centre de la Boh vine, p. 37, pi. 5, fig-s. 1-25 ; 
pi. 6, fig-s. 1-5; pi. 7, figs. 3-9, 12, 13; pi. 241, figs. 6-8; and pi. 242, figs. 2-10. 1867. 
Shell discoid, gibbons; the thickness, as compared with the lateral diameter, 
is quite variable, being ordinarily as one to two, and more rarely as one to 
three. The condition is usually such as not to admit of exact measurement. 
Volutions rounded exteriorly, about six or more, all exposed in the wide 
umbilicus. Transverse section concavo-convex, nearly semicircular, with 
the base concave for the reception of the exterior of the next inner volution. 
The enlargement of the volutions is very gradual. In a specimen of ordinary 
form, a single volution increases from a diameter of ten mm. to a diameter 
of eighteen mm. A similar measurement in another specimen gives an 
increase from nine mm. to seventeen mm. in a single volution. 
Chamber of habitation expanding scarcely more rapidly than the preceding 
volution, occupying a length of something more than one turn of the spire, 
and so far as can be determined, about one volution and a quarter. The 
aperture has the same form as the section of the volutions, and in some 
specimens is apparently a little expanded. Air-chambers somewhat regular 
in the earlier volutions, but unequal in depth near the chamber of habita¬ 
tion ; their depth in some cases being nearly equal to the ventro-dorsal 
diameter of the volution, and varying from four to six mm. between points 
where the tube has increased from a diameter of six to eight mm. 
Septa thin, somewhat regularly and moderately concave, describing a 
gentle curve from the umbilical margin, and limiting a scarcely defined lobe; 
thence bending forward over the peripheral margin, they describe a curve 
which gives a broad,- rather prominent saddle, the summit of which is 
half-way between the margin and the centre of the periphery; thence 
turning acutely backward, they subtend a narrow, acute, ventral lobe, which 
penetrates more than half the depth of the preceding air-chamber. The 
condition of the specimens is usually such that the course of the septa appears 
