138 INFERIOR OOLITE AMMONITES. 



The species of Branch B, on the other hand, are more developed in the 

 Margaritatus -zone than is Hildoceras in the Commune-zone ; while in the Falci- 

 ferum-zone they are more developed than are the highest forms of the Branch A 

 (Ludwigia) in the Murchisonce-zone. At the same time they may have left the 

 parent-stem later than the Branch C, but may have developed quicker. The 

 first forms of Branch B which I have been able to recognise are Am. boscensis 

 and Am. Curioni, which both exhibit the commencement of the falciform or 

 sigmoidal ribbing, and have a suture-line which is more developed than that of 

 the earlier forms of Branch A. The forms from which these two Ammonites are 

 descended ought to be something very nearly like Am. algovianus. Although the 

 involute variety of Am. boscensis (Meneghini, ' Foss. du Medolo,' pi. ii, fig. 18) 

 bears marks of its descent in the furrowed ventral area, yet its ornamentation and 

 sutures lead us on at once to the genus Harpoceras in which we meet with only 

 very rudimentary furrows in one or two species. But in Harpoceras the curvature 

 of the ribs is more highly developed ; the suture-line is more complicated and 

 produces larger, more branched lateral lobes ; the involution is variable, as if in 

 a transition state, as, for example, from the evolute Harp. Strangeivaysi 1 to the 

 involute Harp, subplanatum ; while, probably from this cause, the auxiliary lobes 

 are not much more developed. We, however, meet with a large accessory lobe in 

 the siphonal saddle, and it is not difficult to see the probable reason for its develop- 

 ment. In Arietites and in the less developed Hildoceratidce, that part of the whorl 

 which is least supported by the sutures is between the siphonal and superior-lateral 

 lobes ; but in these quadrangular-whorled forms, this part is in the shape of an 

 arch whose bases rest upon these said lobes. In consequence, the crown of the arch 

 would not require much support ; but, as compression proceeded, this part would 

 become flatter and flatter, therefore less able to stand pressure, and consequently 

 an accessory lobe would be introduced to aid in its support. 



The theory which Dr. Haug puts forward, that Ludwigia is derived from 

 Harpoceras falciferum? seems to me untenable. It leaves the coarse primary ribs 

 unaccounted for; it means that, as the descendant of Harpoceras, Ludwigia is 

 developed in a less degree as regards suture-line, and that in place of the strongly 

 falciform ribs, it has taken on recurved, somewhat angular ribs. (The shape of 

 these ribs is explained if we imagine its descent from Lillia ; the difference in the 

 ribbing of Ludwigia and Lillia is that the ribs in the former spring from a coarse 

 primary rib instead of from a small knob on the inner margin, as in the latter.) 3 



1 Non Am.falcifer, Sowerby ; non Am. serpentinus, Reinecke. The original specimen is in the 

 British Museum, and was badly figured by Sowerby, — the umbilicus is quite one-fourth too large, 

 the inner margin being exhibited concave instead of almost upright and straight. 



2 " Ueber die genetiscben Beziehungen der Grattung Harpoceras," ' Neues Jahrbuch fur Mineral- 

 ogie, Ac.,* Bd. ii, p. 174, 1885. 



3 The coalescing of two ribs on the inner area of Harpoceras might produce something similar in 



