LIOCERAS CONCAVUM, VARIETIES. 73 



moidal or subfalciform ribs, and the other, at any rate during the earlier stages of 

 growth, the V-shaped ribs. The amount of variation which the species exhibits 

 seems to be endless ; yet one fact should be noticed, namely, that if we collect 

 enough specimens we find the variations must have been constant for some definite 

 period, and therefore we can group two or three specimens with one form, half- 

 a-dozen with another, a dozen with a third, and so on ; and both young and old 

 of each of these various forms agree exactly among themselves, but differ from 

 the type in certain features. 



I cannot attempt to mention all these different forms, but will content myself 

 with a few remarks upon the commoner, or the more striking, in the following 

 manner : 



I. Forms with Falciform Ribs. 



(PI. II, figs. 6, 7; PI. VIII, figs. 1, 2.) The typical form (Sowerby's), 

 pp. 56—62. 



a. 1 (PL VIII, figs. 3, 4.) Variety with finer ribs. Bradford Abbas ; scarce. 



j3. Ribs intermediate between those of the type and a ; umbilicus much smaller. 

 Bradford Abbas ; but chiefly found at Louse Hill ; also at Corton Downs, 

 Somerset. 



y. (PI. X, figs. 3, 4.) Thicker ; umbilicus small but deep ; ventral area 

 broader and flatter. PI. VIII, figs. 5, 6, is probably the adult form. Bradford 

 Abbas; scarce. 



8. (PL XII, figs. 1, 2; Lioc. pingue.) Still thicker; fine ribs; very slight 

 carina ; a form somewhat isolated. Peculiar to Halfway House and Louse Hill. 



e. Allied to the type ; umbilicus a trifle smaller ; ventral area and carina more 

 acute. Scarce ; Bradford Abbas. 



I. (PL X, figs. 1, 2; Lioc. formosum.) Rather smooth; umbilicus larger than 

 type ; ventral area acute ; inner area more compressed. Scarce ; Bradford 

 Abbas. 



II. FORMS WITH V-SHAPED RlBS. 



1. Carina fairly prominent. 



r?. Like the type in shape ; ribs fine and numerous ; ventral area slightly 

 flatter. Fairly frequent. Bradford Abbas, Dorset ; Dundry, Somerset. 

 0. Thicker ; ribs coarser. Fairly frequent. Bradford Abbas. 

 i. Smaller umbilicus. Bradford Abbas, Dorset ; Dundry, Somerset. PL VIII, 

 figs. 7, 8, is perhaps the same, but it has peculiar small knobs on middle of lateral 

 area. 



1 This does not refer to Plate VI, var. A. 



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