80 INFERIOR OOLITE AMMONITES. 



.also much too prominent to be in keeping with those forms, and its ventral area 

 is too acute. For my part I should prefer to consider it more nearly allied to 

 them than to Lioc. giganteum but that its immediate ancestor has yet to be 

 discovered. 



The specimens of this species which I possess are all probably nearly adult, 

 and came from the Concavum-beds at Bradford Abbas, Dorset. The largest 

 measures seven inches in diameter, and possesses but little of the body- 

 chamber. The specimen figured (PI. XIV, figs. 10, 11) is just under this measure, 

 but possesses most of the body-chamber, as the position of the cross shows. The 

 test on the side drawn in the figure is not very well preserved, being somewhat 

 decomposed, and some of the characters of the ribbing have had to be delineated 

 by observation of the test on the other side. The specimen possesses, as is shown 

 in the figure, a very thick test. 



The genus Lioceras, the account of which is now brought to a close, is very 

 interesting because of the number of closely-allied species which it produced, 

 and because of the manner in which those species varied. Roughly speaking, 

 the genus may be divided into three portions, although it would perhaps be impos- 

 sible to draw any very marked distinctions between them. They are as follows : 



1. The smooth Liocerata : — Lioc. opalinum, Lioc. decipiens, and their varieties. 



2. The ribbed Liocerata : — Lioc. elegans (Young), 1 Lioc. bradfordense, Lioc. 

 concavum, Lioc. fallax, Lioc. apertum, and their varieties. 



3. The doubtful Liocerata : — Lioc. ambiguum and its variety. 



The last division contains forms smooth or irregularly ribbed, and with a suture- 

 line which is different (p. 29, &c). I fancy it is quite possible they may belong 

 to a different genetic series, and may be an instance of convergence. A species 

 not yet recognised as British, namely, Am. opalinoides, Ch. Mayer (taking the 

 reference to Zieten's fig. 1, pi. vi, { Verstein. Wurtt.,' as the type) may be another 

 species which belongs to division 3. 



One especially singular feature which this genus brings to our notice is that 

 in Lioc. apertum it would appear that the Y-shaped ribs are continued without 

 change. This is altogether contrary to the usual rule, which is that the young of 

 a species resemble the ancestor, while the adults exhibit the greater change. At 

 this rate it would be correct to derive Lioc. v-scriptum from Lioc. apertum, which 

 seems entirely opposed to what we know about the relationship of the other 

 species of the genus. 



1 Thia species occurs in the lowest zone of the Upper Lias and was fully treated of by Dr. "Wright, 

 ' Monograph Lias Amm.,' p. 447. It must be borne in mind that it is entirely distinct from Sowerby's 

 pecies ; but see p. 38. 



