274 THE LIAS AMMONITES. 



The spire is composed of from eight to ten whorls, with convex costated sides, nearly 

 all fully exposed ; the outer whorl has a strongly developed tricarinate area. 



Mouth-aperture nearly quadrate, with rounded angles. The septa, according to 

 d'Orbigny, are symmetrica], foliated on each side, and divided into two or three lobes 

 formed of nearly equal branches. The siphonal lobe is much longer and as wide as the 

 principal lateral lobe, formed of a single narrow branch with four double digitations. 

 Siphonal saddle much larger than the principal lateral lobe, and unequally divided by an 

 accessory lobe (PL II, fig. 3). Principal lateral lobe formed nearly of symmetrical 

 parts, the external branch sometimes provided with acute elongated digitations. Lateral 

 saddle narrow, irregular and oblique. Inferior lateral lobe provided with two unequal 

 branches. Sometimes there is a small auxiliary lobe. A line extended from the extre- 

 mity of the dorsal lobe to the centre cuts three branches of the principal lateral, and 

 passes above the three auxiliary lobes. 



The following important observations were made by the late M. d'Orbigny, 1 on the 

 development of A. Conybeari : 



" This species is smooth only up to the diameter of 2 millimetres — rarely it remains 

 so up to 3. It afterwards develops ribs similar to those of adult age, and has a small 

 keel without lateral sulci; at the diameter of 12 millimetres it has often thirty-six ribs; at 

 the diameter of 19 millimetres forty-six. In others this number is less, and that 

 upon the less compressed specimens with large whorls. Upon these, for example, at the 

 diameter 55 millimetres, there exists only thirty-five to forty ribs ; upon individuals of 

 100 millimetres to 198 millimetres there were sixty-six ribs. In all the cases the lateral 

 sulci on each side of the keel were not developed on the specimens observed up to the 

 diameter of 30 millimetres. These differences in the number of the ribs, and in their 

 proximity or separation, seem to appertain to the sexes, the shells with numerous ribs 

 being probably those of the males. The lobes from early age have very nearly the same 

 form. From the examination of a very large specimen it appears that the ribs disappear 

 in extreme old age." 



Affinities and Differences. — This species resembles A. Kridion, HehL, from the Lower 

 Lias, and figured in Zieten's ' Versteinerungen Wurtternbergs,' tab. hi, fig. 2, and in 

 d'Orbigny, ' Paleontologie Francaise,' Ter. Jur., tab. xxi, figs. 1 — 6. It is distinguished, 

 however, from A. Conybeari by having fewer ribs, and these being more sharp and straight, 

 and terminating in a prominent process near the back ; the keel is more acute, and there 

 are neither lateral sulci nor carina? ; the siphonal and lateral saddles have likewise a different 

 form, and the phases of development of A. Kridion, according to d'Orbigny, are very 

 different from those of A. Conybeari, the shell of the former remaining much longer in 

 the embryonic state without ribs. 



Locality and Stratiyraphical Position. — This is a very characteristic shell of the 

 Lower Lias. I have collected it from the Zone of Arietites Bucklandi in the deep 

 1 ' Paleontologie Francaise, Ter. Jurassique,' torn, i, p. 204. 



