448 l' A UBONTOLOGY OF XEW YORK. 



GONIATITES PLEBEIFORMrS, 11. sp. 



II Ml- XVI. II!.- H,tt;  \. KM. S. 3-9. 



Porn Ilia t rotatoria. Ham.. Illustrations of Devonian Fossils. : Gasteropoda, pL 16, fig*. '-'."). 96. 1876. 

 Compart- Soniatttm pHrius, Barram>k. 1865. Syst. Silur. du centre ile la Bohiiup, p. 37, ]>l. 5. figa. 1 -25 ; 

 pL ti. titf*. 1-D ; pi. 7, Of*. 6-9, 19, 13 : pi. 941, Iga. 6-8 ; and pi. '242, ti{,'s. 2-10. 1867. 



Siiki.i, discoid, gibbous; tbe 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 

 hall-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 



