w 
NArXILID-E. 55 
of two species of a Goniatites \ The inner whorls being broken 
away, the shell appeared to have separated whorls. The genus 
Gifroceras was thus based upon an accident in no way connected 
with the structure of the shell, and a name thus bestowed must 
inevitably have lapsed if it had not been applied to other species of 
fossil Cephalopods to which it was appropriate. Accordingly we 
find the genus Gyroceras taken in hand by de Koninck in 1S44, and 
again in 1880, and restricted to such forms as could he differentiated 
from allied genera. 
8ILUKIAX SPECIES. 
Gyroceras alatum, Barraude. 
1848. Gyrocei'os aJatum, Ban-ande, Ilaidinger, Berichte iiber die Mit- 
theU. von Freund, der Naturwiss. in Wien, Band iv. p. 208. 
1852. Gyroceras alatum, Giebel, Fauna der Vorwelt, Band iii. Abth. i. 
p. 196. 
1867. Gyroceras alatum, Barrande, Syst. Sil. de la Boheme, vol. ii. 
pt. i. p. 162, pi. xhv. ff. 8-18, pi. ciii. ff. L>-20, pi. ccccLxv. ff. 10- 
12, pi. cccclxxvi. caseix. 
1876. Gyroceras alatum, Ferd. lioemer, Ltlhaea Geognostica (Letli. 
Palseozoica, Theil i,). Atlas, Taf. xv. f. 4. 
1883. Hercoceras alatum, Hyatt, I’roc. Boston Soc. Nat. Hist, vol.xxii. 
p. 283. 
N^>. Char. Shell coiled, and usually forming three nearly complete 
whorls, which have a central vacuity, as in many Palaeozoic Xautilidic. 
The whorls are never in contact, but the degree of the separation 
varies in different individuals, and it is greater in the region of the 
body-chamber. The curvature is, however, always maintained in 
Gyroceras, and thus contrasts with the straight termination of the 
shell in LituHes. The transverse section in the present species is 
elliptical, in which the transverse is to the ventro-dorsal axis as 
4 : 3, and sometimes as 5 : 4. The increase in diameter is in the 
ratio of 1 : 3, in the length of the last whorl. The body-chamber 
occupies about half the outer whorl. The aperture has a shallow 
sinus in the centre of the convex border, and on each side there is 
a considerable lamellar expansion of the test forming wing-like 
projections, which are often an inch in length, hence the name 
alatum. The traces of these expansions are seen upon the shell at 
pretty regular intervals, marking successive lips, and forming a 
characteristic feature in its ornamentation. The distance between 
^ This was afterwards described as Goniatites compressus by Beyrich and other 
authors; but Giebel (Fauna der Vorwelt, Band iii. Abth. i. p, 481) restored to 
it its first specific name — Goniatites gracilis. 
