237 



narrow branch with four double digitations. Dorsal saddle miich larger 

 than the superior lateral lobe, and unequally divided by an accessory 

 lobe (pi. 3, fig. 3 a.) Superior lateral lobe formed of nearly symmeti-ical 

 parts, the extei-nal branch sometimes provided with acute elongated 

 digitations. Latei-al saddle, naiTow, irregulai". Inferior lateral lobe, 

 proN-ided with two unequal branches. Sometimes there is a small 

 auxiliary lobe. The line of the central ray setting out from the extremity 

 of the dor.sal lobe passes beneath all the other lobes. 



The following important obsei'vations were made by the late M. 

 d'ORBiGNY,* on the development of A. Conybeari : 



" This species is smooth only vip to the diameter of 2 millimetres — 

 rarely it remains so up to 3. It afterwards developes ribs similar to those 

 of adult age, and has a small keel without lateral sulci ; at the diameter 

 of 12 millimetres it has often 36 ribs ; at the diameter of 19 millimetres, 

 46. In others this number augments less, and that upon the less com- 

 pressed sjjecimens with large whorls. Upon these, for example, at the 

 diameter of 55 millimetres, there exists only 35 to 40 ribs ; upon 

 individuals of 100 millimetres to 198 millimetres, there were 66 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 tlie ribs and 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 veiy large specimen it appears that ribs 

 disappear in extreme old age." 



Affinities a7id Differences. — This species resembles -.4. Kridion, Held., 

 from the Lower Lias, and figured in Zieten's Petrifact. de Wiirtemburg, 

 tab. 3, fig. 2, and in d'OBBiGNY, Paleontologie Francaise, tab. 21, fig. 1-6. 

 It is distinguished however from A. Conybeari by having fewer ribs, and 

 these being more sharp and sti-aight, and terminating in a prominent 

 process near the back ; the keel is more acute, and there are neither 

 lateral sulci nor carinse ; the doi-sal and lateral saddles have likewise a 

 difierent foiin, and the phases of develoi^ment of A . Kridion, accoriling 

 to d'ORBiGNY, ai-e very different from those of A. Conybeari ; the shell 

 of tlie former remaining much longer in the embryonic state without ribs. 



Locality and Stratigra2yhical Position. — This is a very characteristic 

 shell of the Lower Lias. I have collected it from the zone oi Ammonites 

 Bucklandi in the deep cuttings of the Bristol and Birmingham Railway 

 near Bredon, at Fretherne on Severn, Coombe HUl, and several other 



* Palfeontologie Francaise ter. Jurassiqiie, torn. 1, p. 204 



