298 THE LIAS AMMONITES. 



Arietites raricostattjs, Zieten. PL VII, figs. 2 — 6 ; PL XXVI, figs. 5 — 14. 



Ammonites raeicostatus, Zieten. Versteiner. Wurtemb.,p. 18, tab. 13, fig. 4, 1830. 



— — Be Mandelsloh. Const, geolog. Alb. Wiirtemb., 1835. 



— — d'Orbigny. Terr. Jurass., torn, i, p. 213, pi. 54, 1842. 



— CAETJSENSis, eCOrbigny, Terr. Jurass., i, p. 284, pi. 84, figs. 3 — 6, 



1842. 

 Tdeeilites Boblayei, d^Orbigny. Terr. Jurass., i, p. 178, pi. 41, 1842. 

 Ammonites karicostatus, Quenstedt. Motzgeb. Wiirtemb., p. 159, 1843. 



— — Quenstedt. Petrefactenkunde, p. 83, tab. 4, fig. 3. 



— — Fraas. Wiirtemb. Jahresb., ii, p. 210, 1846. 



— — Bronn. Gesch. d. Natur, iii, p. 57, 1847. 



— Ctiebel. Geogn.Septentr. He^cyn. fast, eonstit., p. 19, 1848. 



— — Hehl. Geogn. Wiirtemb., p. 218, 1850. 



— — Schafhdutl. Siidbayr. Alpen, p. 138, 1850. 



— — Renevier. Bull. Soc. Vaud. Sc. Nat., t. iii, p. 139, 1852. 



— — Studer. Geologie der Schweiz, ii, p. 30, 1853. 



— — Von Hauer. Cephalopod. aus dem Lias N.-O. Alpen, 



p. 52, pi. xvi, figs. 10—12, 1856, 



— — Quenstedt. Der Jura, p. 105, pi. xiii, figs. 16—18, 1858. 



— exoetus, Simpson. Foss. of Yorksh. Lias, p. 44, 1855. 



— eaeicostatus, Bumortier. Depots Jurass. du Bassin du Rh6ne, tome ii, 



p. 173, pi. XXV, figs. 4—7, 1867. 

 Aegoceeas? eaeicostatum, Tate and Blake. Torksh. Lias, p. 276, 1876. 



Diagnosis. — Shell compressed, discoidal ; whorls narrow, numerous, almost entirely 

 evolute ; sides with 21 to 30 straight, acute, prominent, and distant ribs, diminishing 

 in size on the siphonal area, which is flat, with a small, narrow, rudimentary keel, without 

 lateral sulci ; aperture quadrate, rotundo-cordate. 



jDiviensions. — The large specimen figured in PL VII, figs. 2, 3, 4. Diameter 93 

 millimetres; ditto of umbilicus 56 miUimetres; height of aperture 20 millimetres j width 

 of ditto 18 millimetres. 



The smaller specimen figured in PL XXVI, figs. 12, 13, 14 : diameter 60 milli- 

 metres; ditto of umbilicus 35 millimetres; height of aperture 15 millimetres; width of 

 ditto 15 millimetres. 



Dimensions of the French specimen measured by A. d'Orbigny : diameter 93 milli- 

 metres ; size of the last whorl -j^ ; amount of involution y^q- ; diameter of the 

 umbilicus -j^; thickness of the last whorl -^^. Another, measured by M. Dumortier, 

 gave: diameter 65 millimetres; size of the last whorl -yoo; thickness i^; size of 

 umbilicus -j^. In a smaller shell of 35 millimetres all the other proportions remain 

 the same. 



BescrijMon. — This is an important and very characteristic Ammonite, as it marks 



