260 THE DEVONIAN OF MISSOURI. 



* 



Description— -Shell of medium size, spirally ascending, sub-conical in outline; apex 

 consisting of two or three closely coiled but free volutions. Body whorl expanding 

 regularly, and towards the aperture somewhat rapidly; aperture almost circular and 

 rather broadly expanded; character of peristome obscure. Fine, undulating, and closely 

 crowded transverse striae mark the surface. 



Remarks — This species is represented by some small specimens which are ap- 

 parently young individuals, and by several incomplete large forms which are doubt- 

 fully referred to the same species. They display a considerable variation in the rapidity 

 of expansion of the body whorl, and in the amount of expansion of the aperture, but in 

 general they closely approximate the above species. 



Horizon — -Lower beds. 



Figured specimens No. 27530, Walker Museum. 



Platyceras retrors'um Hall 



Plate 69, figure 10 



1859. Platyceras retrorsum Hall, Nat. Hist. N. Y. Pal., p. 320, pi. 58, figs. 10a, b, 6c; 

 pi. 59, figs. 9a, b. * 



Description — Shell of medium size, depressed, ascending spirally, whorls about two, 

 free; the first whorl nearly smooth, while the second or body whorl is marked with widely 

 spaced, spiral ridges or corrugations, which are absent on the ventral side of the shell. 

 Dorsal side flattened, the apex a little elevated above the outer volution of the shell. 

 Surface marked transversely by fine, closely spaced, sinuous striae. Aperture not 

 definitely 'known, probably subcircular with undulating peristome. 



Remarks — One exfoliated individual occurs which has portions of the shell attached. 

 Apart from its smaller size, there seems to be no essential difference from the specimens 

 in the Lower Helderberg of New York. 



Horizon — Lower beds. 



Figured specimens No. 27524, Walker Museum. 



Platyceras cf. subconicum Ohern 

 Plate 69, figure 11 



1913. Platyceras subconicum Ohern, Geol. Surv., Low. Dev., p. 479, pi. 86, figs. 1-3. 



Description — Shell small, obliquely conical, whorls probably one; shell expanding 

 fairly rapidly from apex to aperture; apex broken but it appears to have been blunt and 

 twisted slightly to the right. Aperture and peristome obscure; surface features not 

 preserved. 



Remarks — The specimens are entirely too fragmentary for certain determination, 

 and the reason for referring the specimen to this species is because of the evident cor- 

 respondence in shape with authentic specimens in the collection of Walker Museum. 



Horizon — Lower beds. 



Figured specimen No. 27520, Walker Museum. 



Platyceras trilobatum Hall 



Plate 69, figure 12 



1859. Platyceras trilobatum Hall, Nat. Hist. N. Y. Pal., vol. 3, p. 316, pi. 57, figs. 5a, 



b, c, 1861. 



1913. Platyceras trilobatum Ohern, Geol. Surv., Md. Low. Dev., p. 475, pi. 84, figs. 7, 8. 



Description — "Body of shell obliquely or arcuately ovoid, trilobate; volutions 



three or four, the last one (or more) becoming free, gradually expanding to the aperture; 



the apex closely involved and rising above the plane of the outer volution, or sometimes 



