LYTOCERAS LINEATUM. 409 



whilst they are more convex and laterally inflated in Lyt. cormicopia. D'Orbigny has 

 further observed that in Lyt. fimhriatwn there are only two lateral lobes, whilst in Lyt. 

 cornucopia there are three lateral lobes. To these differential characters it may be added 

 that the shell is deeply ornate, and has a series of longitudinal stride which intersect at 

 right angles the transverse striae, but produce a less regular square-celled structure than 

 in the shell of Lyt. cornucopia. The wing-like elevations are more numerous and less 

 elevated in cornucopia than they are m fimbriatum. 



Locality and Stratiyraphical Position. — Lyfoceras fimbriatum had a considerable 

 range of distribution in time during the deposition of the Middle Lias. I have collected 

 this Ammonite in the Aey. Jamesoni, Aeg. Ilenleyi, and Amal. margaritatus beds in the 

 Counties of Gloucester, Somerset, and Dorset. The finest specimens were those collected 

 near Charmouth, from a sandy stratum, associated with Amaltheus margaritatus. 



Lytoceras LINEATUM, ScJilotJieim, PI. LXIX, fig. 1. 



, Ammonites lineatus, Sehlotheim. Petrefactenkunde, p. 75, No. 24, 1820. 



— TENUICOSTA.TUS, Young and Bird. Yorkshire Coast, p. 253, 1828. 



— LINEATUS, Quenstedt. Flbzgebirge Wiirtembergs, p. 171, 1843, 



— — — Cephalopoden, p. 102, tab. vi, figs. 8 a, b, 



1849. 



— — Giehel. Fauna der Vorwelt, Bd. iii, p. 405, 1852. 



— TENUicosTATUS, SimpsoTi. Yorkshire Lias, p. 40, 1855. 

 Lytocekas LINEATUM, Tdte and Blake. Yorkshire Lias, p. 298, 1876. 



Diagnosis. — Shell discoidal, volutions five, all exposed, rapidly diminishing in size ; 

 outer whorl two fifths the diameter of the shell ; numerous fine, hair-like ribs, commencins: 

 at the suture, inclining gently towards the aperture, passing across the broad, round, 

 siphonal area, and uniting with the ribs from the opposite side ; the fine ribs are feebly 

 fimbriated, and there are two wing-like elevations on the last whorl. 



Dimensions. — Transverse diameter 168 millimetres; width of umbilicus 62 milli- 

 metres ; height of aperture 70 millimetres. 



Description. — This Ammonite possesses a leading feature, which is well expressed in 

 the list of synonyms given above, but whether the numerous, slender, annular ribs with 

 fimbriated margins encircling the whorls constitute a character of sufficient importance 

 to differentiate a species distinct from Lyt. fimbriatum^ with which it has so many points of 

 structure in common, may well be doubted, I would prefer considering Lyt. lineatum as 

 a small-ribbed variety of the typical Lyt. fimbriatum I have described in detail, and of 

 which I have figured several very beautiful examples. If Sowerby's type figure (' Mineral 

 Conchology,' vol. ii, pi. 164) be compared with PI. LXIX, fig. I, it becomes evident 



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