THE CEPHALOPODA. 



189 



Aegoceras capricornum, Sckloth (fig. 56). 



— catenatum, Sow. 



— Charmassii, d'Orbig, 



Aegoceras Henleyi, Sow. 



— planicosta, Sow. 



— planorbis, Sow. 



V. Planulati. — The siphonal area is always perfectly round, and is joined to 

 the sides without a border. The whorls lie almost on the same plane, whereby all 



Fig. 57. — Stephancceras annulatum Sow. 



Fig. 58. — Stephanoceras commune, Sow. 



the species receive a more or less strikingly discoidal form. The numerous close-set 

 ribs divide near the half or two thirds of their height into two, three, or many small folds, 

 without any tubercle at the point of bifurcation. The lower portion of the rib is 

 thicker and directed backwards, as in the modern genus Siep/ianoceras, which is the type 

 of this group. The disposition of the lobes is very peculiar and complicated in the 

 Planatuli. The upper lateral is large and very deeply sunk, and occupies the middle 

 of the side ; the lower lateral is smaller, and there proceeds from it two or three 

 auxiliary lobes with branches directed obliquely backwards or sometimes even horizontal, 

 and these form together a compound lobe larger than the lower lateral, under which it 

 retires, the whole constituting a most intricate arrangement which is very difficult to trace 

 out. 



The Planulati are abundant in the Middle Oolites and in the Upper White 

 Jurassic Limestone of Germany. They are also found in some of the beds of the Upper 

 Lias. They are entirely absent from the Chalk. The following forms are characteristic 

 of this group. 



Perisphinctes biplex. Sow. 



— giganteus, Sotv. 



— plicatilis, Sow. 



— polygyratus, Schloth. 



Perisphinctes polyplocus. Rein. 

 Stepbanoceras commune. Sow. (fig. 58). 



— annulatum, Sow. (fig. 57). 



— crassum, Young and Bird. 



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