26 INFERIOR OOLITE AMMONITES. 



on the other, might lead a superficial observer, taking no account of ribbing or 

 suture-line, to suppose that the derivation of Lloccras concavum from Ludwigia 

 Murchisonce could be demonstrated. "We know, however, that the genus Lioceras 

 began, at any rate in the Upper Lias, with. Lioceras elegans, Young (not Harpo- 

 ceras elegans, Sowerby), 1 and Lioceras elegans is naturally the most probable 

 progenitor of the other species of this genus. 



From Lioceras concavum this variety is easily separable by its greater thickness, 

 and its umbilicus showing portions of the inner whorls throughout all stages of 

 growth ; from the thicker forms of Lioceras v-scriptum the difference in ribbing, 

 sigmoidal instead of V-shaped, always distinguishes it, in addition to the other 

 features, or even when some of the smaller specimens of Lioceras v-scriptum have 

 a tendency to depart from the ordinary concave umbilicus (Plate X, fig. 7). The 

 young of this form have a great resemblance to a small variety of Lioceras concavum, 

 viz. Lioc. pingue, which, however, possesses finer ribs and continues always with 

 a concave umbilicus (Plate XII, fig. 3 and fig. 7, show the difference in this 

 matter). Lioc. pingue also never attains a much larger size than the specimen 

 figured. Lioceras giganteum occurs in the Murchisonai-zorie at Beaminster with 

 specimens of Lioceras bradj vr -dense. I do not know of its occurrence in the Brad- 

 ford Abbas district. 



Plate XI, fig. 1, shows the side view of a very large adult specimen with the 

 greater portion of the test in a fair state of preservation. A large part of the 

 specimen has not been placed upon the plate, as it shows no different character. 

 The last suture is situated at the X . This specimen is from Beaminster and is 

 in my collection. Plate XII, fig. 4, gives the front view of this specimen. The 

 test is present on the ventral area, and is seen on the top of the figure. Plate 

 XII, fig. 5, shows a young specimen from Stoke Knap, near Broad Windsor, 

 Dorset, in my collection ; fig. 6 displays the front view ; and fig. 7 a sectional view 

 of the umbilicus to compare with fig. 3. 



Lioceras ambiguum, S. Buchman. Plate VII, figs. 1 — 6. 



The Type. — Discoidal, much compressed, subcarinate, whorls merely orna- 

 mented with very fine sigmoidal lines of growth, which at an earlier stage of the shell 

 have exhibited a slight tendency to form very fine ribs. Whorls distinctly convex, 

 forming a slight ridge; this is situated nearly in the middle, and gives to the 

 inner portion with the umbilicus, a regularly depressed, plate-like appearance, only 

 relieved by another slight ridge on the edge of the inner margin. The ventral area 

 is narrow, and slopes towards a small, scarcely distinct carina ; but on the body- 

 ] See "Jurassic Ammonites," S. Buckman, ' Geol. Mag.,' 1887, dec. ii, vol. iv, No. ix, pp.398 — 100. 



