420 THE LIAS AMMONITES. 



are very plainly visible. None of the shells I have examined have the suture-line pre- 

 served ; and, as Prof. d'Orbigny had examples showing this character, I give the 

 chief points of his description. " The septa are symmetrical, divided on each side into six 

 lobes and saddles formed of unequal parts. The siphonal lobe is much shorter than the 

 principal lateral lobe, formed of three branches, of which the terminal one ends in three 

 twigs. The siphonal saddle is as large as the principal lateral lobe, and terminates in five 

 oval, spatuliform leaves. The principal lateral lobe is long and ornate ; it has tlu'ee 

 branches without and two within, all very unequal. The second lateral has much the 

 same figure as the principal lateral, but does not attain to one half its size. The three 

 auxiliary lobes have irregular branches, and the three auxiliary saddles terminate in oval 

 leaflets." 



D'Orbigny likewise observed that " A. Loscomhi is a very variable species. The 

 mould is in general smooth, nevertheless some specimens are found, considered to be the 

 young of this species, with slightly marked transverse constrictions very closely approxi- 

 mated. It is, perhaps, one of the forms which exhibits the most marked variation 

 at different ages. Up to the diameter of 5 millimetres its wdiorls are only one half 

 involute, the umbilicus is very wide, and the shell ornamented transversely by a spiral 

 revolution of six, deep, transverse, oblique constrictions. These constrictions and this form 

 continue for a longer or shorter time in diff'erent individuals ; sometimes the constrictions 

 disappear and the whorls enlarge up to the diameter of 8 millimetres, although in others 

 this state continues up to the diameter of 20 millimetres : this is the extreme limit observed 

 in which the shell preserves the ornamentation of youth." Terr. Jurass., tom. i, p. 265. 



M. Dumortier found a fragment of this species 190 millimetres in diameter at 

 Mazaugues, Var; which carried its lobes up to the last extremity, proving that it had 

 attained to much larger dimensions. The shell was in part preserved, and permitted 

 him to see the small lines radiating very equally and regularly and forming an elegant 

 ornamentation ; these lines, slightly flexed, are strongly inclined forward, and afterwards 

 describe a round sinus on the area where they preserve all their value " (' Depots Juras- 

 siques,' tom. iii, p. 78). 



Affinities and Differences. — This species, both in its form and the structure of the 

 suture-line, closely resembles the Phylloceras heteroplyllum ; it is readily distinguished 

 from that species by its open umbilicus, the structure of the lobes, and by the siphonal 

 lobe being formed of single elements, whilst in F/zt/IL heterophyUum they are in pairs. 



Locality and Sfratiyraphical Position. — This Ammonite is a leading Mollusk of the 

 Green Ammonite-bed, the zone of Aey. Davoei in the Middle Lias. I have it in the 

 same zone from Thackwell's brickfields, Leckhampton, and Professor Blake reports it 

 from the same horizon in Robin Hood's Bay and Huntcliff", Yorkshire, where it is rare. 



In Prance it has been collected in the Middle Lias which it characterises at Coutards 

 and near Saint Amand, Cher. ; Mulhausen, Bas-Rhin ; Vieux-Pont and Bayeux, Calvados; 

 Venavey, near Semur, Cote-d'Or ; Saint Helene, Saone-et-Loire ; Mazaugues, Var. 



