312 THE LIAS AMMONITES. 



neither marks of striae nor the merest rudiment of a keel on the cast (figs. 3 and 4). I 

 have not seen the lobe-line of this species, and therefore quote d'Orbigny's descrip- 

 tion. The septa are symmetrical, divided on each side into five lobes and five saddles, 

 formed of unequal parts. The siphonal lobe is as large, but shorter than the principal 

 lateral, and its sides are ornamented on each side with four digitations; the siphonal 

 saddle is one third larger than the principal lateral lobe, and divided into three obtuse 

 folioles, the central leaf being the largest. The principal lateral lobe is ornamented on 

 each side with two digitations ; the superior is simple and terminates in a digitation 

 with three points ; the lateral saddle, as large as the principal lateral lobe, is divided into 

 four obtuse unequal folioles ; the inferior lateral lobe, very oblique and irregular, is half 

 the length of, and much narrower than the principal lateral lobe. Besides these there are 

 three oblique auxiliary lobes, represented by as many obtuse points. A transverse line, 

 passing from the extremity of the siphonal lobe, cuts the extremity of the principal lateral, 

 passes below the inferior lateral lobe, touches the extremity of the first auxiliary lobe, 

 and cuts across the others. 



Affinities and Differences. — This Ammonite differs from Aeg . planorbis in possessing 

 more whorls, which are not so deep, and in having the middle of their sides covered with 

 thick, short, blunt ribs, which vanish from the outer and inner margins. It has been 

 considered by Dr. Oppel to be the Aeg. psilonotum plicatmn , Quenst., still it differs from 

 that Ammonite in possessing narrower and thicker whorls, rounder in form, and slower in 

 growth, the sides of which have short, thick, inflated ribs, and a much wider umbilicus. 

 It much resembles Aeg. intermedium, Portlock, from the Lower Lias of Ballintoy, County 

 Antrim (PI. XV, figs. 3 — 6). In this species, however, the ribs are more acute and 

 curve forward toward the aperture (figs. 3 and 4) ; the siphonal area is likewise covered 

 with fine cresentic folds (figs. 5 and 6) ; in other respects the resemblance is very great. 

 The next Ammonite which stands in close relationship with Aeg. Johnstonii is Aeg. 

 BelcJieri, Simp. ; the latter appears to resemble Aeg. torus, d'Orb., or the French form of 

 Aeg. Johnstonii. These ribbed species of the Psilonoti section are very puzzling indeed, 

 and a much larger number of specimens are wanted for comparison than we at present 

 possess before correct determinations can be made out regarding them. 



Locality and iStratigraphical Position. — At Watchet, Aeg. Johnstonii is found 

 crushed flat in the shales of the lowest beds of the Lias. The mother-of-pearl 

 layer of the shell is wonderfully preserved, the fossils shining forth in beautiful iridescent 

 colours when the shales are laid open with the hammer ; there are some fine large slabs 

 covered with these iridescent Ammonites in the British Museum, which all came from 

 Watchet ; the form of the shell, however, cannot be made out from any of the specimens 

 found in this locality. It is the constant associate of Aeg. planorbis, Sow., in British 

 Liassic rocks, just as Aeg. psilonotum plicatum, Quenst., is the associate of Aeg. psilo- 

 notum lave, Quenst., in those of Wiirtemberg. 



