308 THE LIAS AMMONITES. 



Second Section, Angulati. 



Sides covered with sharply-flexed ribs, interrupted on the ventral area by a channel 

 more or less developed. Aeg. angulatum, Schl. ; Aeg. Charmassei, d'Orb. ; Aeg. 

 catenatum, Sow. ; Aeg, lacunatum, Buck. ; Aeg. Boucaultianum, d'Orb. 



All these forms, the two last excepted, are from the Angulatum-zone of the Lower 

 Lias. 



Third Section, Armati. 



Sides with numerous ribs, many of which develop tubercles or elongated spines ; in 

 some forms they are absent. Aeg. Davcei, Sow. ; Aeg. planicosta, Sow. ; Aeg. Birchii, 

 Sow. ; Aeg. bifer, Quenst, ; Aeg. armatum, Sow. ; Aeg. Taylori, Sow. ; Aeg. densinodum, 

 Quenst. ; Aeg. brevispinum, Sow. ; Aeg. Valdani, d'Orb. ; Aeg. Maugenesti, d'Orb. 



Aeg. planicosta, Aeg. Birchii, and Aeg. bifer, are from the Upper Bucklandi-beds of 

 the Lower, and all the others are from the Jamesoni zone of the Middle, Lias. 



Fourth Section, Involtjti. 



Shell highly involute; inner whorls sometimes almost entirely concealed. Shell 

 during middle age often undergoes a great change of form, when the involution becomes 

 less, and the umbilicus widens. Aeg. Henleyi, Sow. ; Aeg. striatum, Reinecke ; Aeg. 

 Bechei, Sow. ; Aeg. curvicornum, Schloenb. ; Aeg. pettos, Quenst. ; Aeg. heterogenes, 

 Young and Bird. 



All the forms of this section are from the Henlegi-zowe of the Middle Lias. The 

 genus Aegoceras began in the Muschelkalk, zone of Arcestes Studeri, with Aeg. incultum, 

 Beyr., Aeg. Pahnai, Mojs. ; and became extinct in the Middle Lias. 



First Section. — Psilonoti. 



Aegoceras planorbis, Soioerby. PL XIV, figs. 1 — 4. 



Ammonites planorbis, Sowerby. Mineral Conchology, vol. v, p. 69, pi. 448, 



1825. 



— ekugatus, Phillips. Geol. Yorksh., pi. 13, fig. 18, 1829. 



— psilonotus, Quenstedt. Flotzgebirge Wurtembergs, p. 127, 1843. 



— erugatus, Simpson. Monogr. of the Ammonites of the York- 



shire Lias, p. 11, 1843. 



— Sampsoni, Portlock. Geology of Londonderry, p. 136, pi. 29, 



fig. 13. 1843. 



— Hagenowii, Bunker. Lias bei Halberstadt ; PakEontograpkica, 



vol. i, p. 1 15, pi. xiii, fig. 22 ; pi. xvii, fig. 2, 1846. 



