236 THE LIAS AMMONITES. 



compressed. Keel crenulated and well-defined. Umbilicus open, whorls exposed, or 

 only partially involuted. Ex. Amaltheus margaritatm, Mont. (PI. XLIX) ; the other 

 forms named are all varieties of Montfort's capital typical species. 



Cycloceratid^. 



This family is remarkable for containing species which, on one side, ally it with 

 LipauoceratidjE, and, on the other, with Hildoceratid^. The form is more com- 

 pressed than the former, and the tuberculations of the pilse separate it from the latter. 



Genus Plattpleuroceras, Hyatt. — Abdomen nearly as broad, or broader than the 

 dorsal side of the whorl. Pilse single, tuberculated, and extending across the rounded 

 abdomen. Septa minutely divided by minor lobes, Ex. Aegoceras latecosta. Sow. 

 (PI. XXXII, fig. 1). 



Genus Cycloceras, Hyatt. — Abdomen rounded or keeled, not so broad as the dorsal 

 side of the whorl. Pilse single, tuberculated, and not extending across the abdomen in 

 the keeled species, young smooth for the first two or three whorls, then becoming ribbed. 

 Ex. Aegoceras natrix, Schloth. Aegocer. Valdani, d'Orbig. (PI. XXXVIII, figs. 1 — 4). 



Genus Tropidoceras, Hyatt. — Abdomen invariably keeled, much narrower than the 

 dorsal side of the whorl. Pilse single, smooth, or tuberculated in the same species, do 

 not extend across the abdomen in any species. Young smooth for one or two whorls, 

 keel and pilse appear simultaneously. Ex. Harpoceras Masseanum, d'Orb. 



Tipper Lias. 



Discoceratid^. 

 Ophioceras Levesquei, d'Orb. = Harpoceras Levesquei, d'Orb. 



Deroceratid^. 

 Deroceras subarmatum, Sow. = StepJianoceras suharmatum, Sow. 



Dactyloid^. 

 Cceloceras Grenouillouwii, d'Orh. = Aegoceras pettos, Quenst. (PI. XXXVII, figs. 5 — 7). 



Genus Dactylioceras, Hyatt. — Abdomen equal in breadth, or less in width than 

 the back, instead of being broader than, or equal in breadth to the back, as in the pre- 

 ceding genera. The lateral pilse in the adult are smooth, and invariably single ; the 

 abdominal pilse may be either bifurcated or single. Ex. Stephanoceras commune, Sow. 



