202 
THE LIAS AMMONITES. 
Genus Helicoceras, d'Orhlgny. Shell turbinoid, spiral, composed of a few 
whorls obliquely coiled and rather widely separated. Spire dextral or sinistral, and 
elevated above the plane of the last large whorl, which was the dwelling chamber of the 
animal. Septa transverse, oblique, and deeply sinuous. Siphuncle marginal. Helicoceras 
Bobertiamis, d'Orb. (fig. 95), forms the type. The species are found in the Jurassic 
and Cretaceous rocks. 
Genus Heteroceras, d' Orhigny. Shell spiral, turreted, whorls contiguous in youth ; 
with age, the last whorl separates from the others and becomes produced and recurved, 
forming an arch without septa, and which doubtless constituted the dwelling chamber of 
the animal. Heteroceras is a Turrilite with the dwelling chamber in the adult shell 
detached, produced, and recurved. The most typical specimen is Heteroceras Emerici, 
d'Orb. (fig. 96). 
Genus Anisoceras, Pictet. Turns of the shell at first spiral not in the same plane, 
next tangentially divergent, and finally recurved, none of the turns in contact. Found 
in Oolitic and Cretaceous formations. 
Professor Pictet,^ in his excellent ' Traite Elementaire de Paleontologie,' gave a 
copious resume oi the Cephalopoda, in which he closely adhered to the system of Von Buch 
and d'Orbigny. 
Professor H. B. Geinitz,^ in his ' Grundriss der Versteinerungen Kunde,' followed 
out the same method. 
Dr. C. G. Giebel,^ in his ' Eauna der Vorwelt,' gave a valuable synopsis of the 
Eossil Cephalopoda, and brought together a great quantity of material bearing upon tliis 
subject, in the arrangement of which he followed the methods of von Buch and 
d'Orbigny. 
The late Mr. Daniel Sharpe, E.R.S.,* contributed an unfinished monograph on the 
British Cretaceous Nautili and Ammonites to the volumes of the Palasontographical 
Society in 1853, 1857. In this important work its learned author followed the hnes 
laid down by d'Orbigny in the systematic arrangement of the groups, and illustrated his 
work with well-executed plates. 
The Jurassic geology of the north-eastern Alps received an important illustration 
from the researches of Eranz Ritter von Hauer,^ whose exhaustive monograph, illustrated 
with beautiful plates, 'Die Cephalopoden aus dem Lias der Nordostlichen Alpen,' marked 
an epoch in our knowledge of the Liassic groups of that region. In this important work 
the learned author followed the grouping of von Buch. 
^ 'Traite elementaire de Pal6ontologie,' torn, ii, p. 309, pi. xiv — xx, 184-^. 
2 ' Grundriss der Versteinerungens Kunde,' pp. 256 — 315, pi. x — xii, 1S46. 
^ ' Fauna der Vorwelt Cephalopoden,' Band iii, 1852. 
■* 'Cretaceous Ammonites,' Palseontogr. Soc, vols, for years 1853 and 1855. 
^ 'Ueber die Cephalopoden aus dem Lias der Nordostlichen Alpen,' 1856. 
