322 THE LIAS AMMONITES. 



Aegoceras Moreanum,^ d'Orhigny. PI. XVII, fig. 1 — 6. 



Ammonites Moreanus, d'Orhigny. Pal^ont. Fran9., Terr. Jurass., p. 299, pi. sclii, 



1842. 



— ANGULATUS, Stur. Jahrbucli der k. k. geolog. Reich., vol. i, p. 165, 1851. 



— MoBEANUS, Studer. Geologic der Schweitz, vol. ii, p. 30, 1853. 



— — von Hauer. Jahrbuch geologischen Eeichsanstalt, vol. iv, 



p. 736, 1853. 



— — Martin. Infra-Lias du Depart, de la Cote-d'Or, p. 38, 1860. 



— — Terquem et Piette. Lias inf. de Test de la France, p. 26, 



1865. 



Diagnosis. — Shell discoidal, compressed ; whorls flattened, sides convex, internally 

 smooth and feebly undulated, externally crenulated, with prominent bifurcations of obsolete 

 ribs ; siphonal area narrow, smooth in the centre, and costated at the outer margin ; 

 aperture compressed, triangular, incised internally by the penultimate whorl, and notched 

 outwardly by the siphonal area ; involution one-half the depth of the whorl. 



Dimensions. — Transverse diameter 170 millimetres; width of the umbilicus 48 milli- 

 metres ; height of the last whorl at the aperture 70 millimetres ; greatest width of ditto 

 43 millimetres. In four specimens before me, including the fossil figured, the relative 

 proportions of these parts vary considerably. Professor d'Orbigny's type specimen mea- 

 sured : — Diameter, 75 millimetres ; in relation to the diameter the size of the last whorl 

 was -j^ ; involution of the last whorl j^ ; width of the umbilicus -^^ ; thickness of 

 the last whorl y^. 



Description. — This Ammonite has a compressed, discoidal shell, with flattened 

 whorls, the sides of which are slightly convex ; on the inner or umbilical half of the whorl 

 they are smooth, and have from thirty to forty curved undulations, representing so many 

 obsolete ribs ; near the outer or siphonal margin the folds appear to bifurcate, and 

 develop a number of short, transverse, oblique costse, more or less prominent in different 

 specimens ; the spire is formed of very compressed whorls, which are nearly one half 

 concealed by the volutions of the spire, leaving a narrow umbilicus. The siphonal 

 area is contracted, smooth in the centre, and crenulated at the sides by the develop- 

 ment of the short oblique costse which ornament the outer margin of the shell. The 

 aperture is compressed, and more or less triangular, and varies in the proportional 

 measure of its height in different specimens. 



The young shells of this form, up to two to three inches in diameter, are pro- 

 vided with simple bent ribs, which terminate near the outer margin ; these ribs become 

 enlarged as they approach the margin, and are sharply bent inwards at the side of the 

 siphonal area, where they disappear, leaving the centre of this space quite smooth. 



^ Considered to be a variety of Aegoceras angulatum and named Aegoceras angulatum in the 

 explanation of Plate XVIL 



