37b YORKSHIRE TYPE AMMONITES Apr. 



37. AMMONITES NAUTILIFORMIS, J. Buckman 

 (Plates XXXVII a, b, c, d.) 



Original Description 

 [J. Buckman in Murchison, Geol. Chelt. ed ii, 1844, pp. 90, 105.] 



" Ammonites nautiliformis (Buckman) L.M. [Lias Marlstone], 

 Dumbleton [p. 90]. 



" A. nautiliformis. — Back rounded, broad; volutions 3 to 4 — the 

 outer volution occupying more than half of the whole diameter, the 

 inner ones only slightly exposed ; aperture large, obtusely cordate ; 

 ribs small, those of the upper part of the cell proceeding from the inner 

 margin to about mid-way across the outer volution, where they are 

 joined to a large tubercle, from which they divide into three smaller 

 ones, which proceed across the back and are united in the same manner 

 on the opposite side. Diameter 8in., thickness sJin. 



" Locality. — Lias Marlstone, Alderton Hill. 



" Only a single specimen, and that somewhat imperfect. Its form 

 is so much like that of the Nautilus that, had not the saddles been well 

 exposed, we should almost have considered it as belonging to the latter 

 genus [p. 105]." 



Remarks 



Stages, conch, sphaerocone ; periphery, i ; ornament, 5**. 



This is the specimen pointed out to me by my father as the type 

 of his A. nautiliformis, and the only example which he had obtained. 

 There is, however, discrepancy in the mesLsurements ; the diameter is 

 6J inches (156 mm.), and the thickness about 4^ inches (113 mm.). 

 The description is also incorrect : There are two lines of tubercles, though 

 they are obscure ; the ribs on the periphery (back) are not smaller than 

 those on the inner marginal area, but larger. 



The saddles (and lobes) are exposed in place.=, but not enough for 

 delineation. 



This specimen, though not a Yorkshire Type, is figured to illustrate 

 remarks on development (p. xiv), and on genera (p. Hi). 



Genus, /Egoceras, Waagen (Gen. p. iii). It is assumed that this 

 species represents the sphaerocone stage of JEgoceras, being distinguished 

 from Androgynoceras (sjihsrocone) by more inflated whorls and coarser 

 ornament, and from Liparoceras by less coarse and more regular orna- 

 ment. Family, Liparoceratidae, Hyatt, 1867. 



Geological position — The term " marlstone " would presumably 

 indicate the strata of the spinatum and margaritatus zones at the locality : 

 the specimen has not the matrix of the former. 



Alderton Hill is an eminence in the parish of Dumbleton. 



Result 



jEgoceras nautiliforme, J. Buckman sp. 1844, Domerian, 

 [margaritattts zone], Alderton Hill, Dumbleton, Gloucestershire. 



Note 



A Somerset specimen is figured (Plates XXXVII c, d) to shew 

 the ornament. The ribs on its venter are somewhat larger, more distant 

 and fewer than those of the iyyie. It is from the Marlstone of South 

 Pcthcrton. 



