202 THE LIAS AMMONITES. 



Genus Helicoceras, d'OrUgny. Shell turbinoid, spiral, composed of a few 

 whorls obhquely 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 

 Bobertianus, d'Orb. (fig. 95), forms the type. The species are found in the Jurassic 

 and Cretaceous rocks. 



Genus Heteroceras, d' Orbigny. 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 Tarrilite 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 re'sumeoithe Cephalopoda, in which he closely adhered to the system of Von Buch 

 and d' Orbigny. 



Professor H. B. Geinitz,^ in his ' Grundriss der Versteiuerungen Kunde,' followed 

 out the same method. 



Dr. C. G. Giebel,^ in his ' Pauna der Vorwelt,' gave a valuable synopsis of the 

 Fossil Cephalopoda, and brought together a great quantity of material bearing upon this 

 subject, in the arrangement of which he followed the methods of von Buch and 

 d'Orbigny. 



The late Mr. Daniel Sharpe, F.R.S.,* contributed an unfinished monograph on the 

 British Cretaceous Nautili and Ammonites to the volumes of the Palseontographical 

 Society in 1853, 1857. In this important work its learned author followed the lines 

 laid down by d'Orbigny in the systematic arJ'angement 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 Franz 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 Paleontologie,' torn, ii, p. 309, pi. xiv — xx, 184-'>. 

 2 ' Grundriss der Versteinerungens Kunde,' pp. 256 — 315, pi. x — sii, 1846. 

 ^ '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. 



