102 INFERIOR OOLITE AMMONITES. 



Ludwigia costosa {Quenstedt). Plate XX, figs. 11, 12. 



1886. Ammonites opalintjs costostts, Quenstedt. Am. Schwabiscben Jura, pi. 



lv, fig. 20, only. 



Discoidal, compressed, carinate ; whorls very slightly subconvex, ornamented 

 with a few bifurcating ribs, which are slightly reflexed, and are conspicuous on the 

 inner area ; ventral area subacute, smooth, carrying a small carina ; inclusion for 

 about one-half of the previous whorl ; umbilicus fairly open, slightly ribbed. 



The scanty material in my possession remained unnoticed until I observed the 

 same form figured in Quenstedt's work. He has considered it a variety of Lioc. 

 opalinum ; but with this I cannot agree. Its suture-line, even according to his 

 figure, shows all the characters of the lobe-line of Ludwigia, and none of those of 

 Lioceras ; while its ribs, conspicuous on the inner area, bifurcate and reflexed, 

 agree thoroughly with the definition of the genus Ludwigia. I have therefore no 

 hesitation in regarding this form as a species with the name Ludwigia costosa. 



From the coarsely ornamented forms of Ludwigia Murohisonce this species is 

 separated by being much thinner ; and from the compressed forms of that species 

 by being more coarsely ornamented ; while from both it differs by its ribs being 

 placed much farther apart, and somewhat irregular. The latter features separate 

 it from the coarser form of Ludwigia cornu, to which it has much similarity. 



The fact that this species occurs in the Opalinum-zone is very important, because 

 it shows that L. cornu and L. rudis are not in the same genetic series as L. Mur- 

 chisonce. At one time I was inclined to the opinion, in the absence of evidence to 

 the contrary, that L. cornu was descended from some of the more finely ribbed 

 forms of L. Murchisonce ; but such evidently is not the case. Ludioigia costosa in 

 the Opalinum-zone is the parent form of L. cornu and L. rudis which occur in the 

 Concavum-heds. 



On the other hand, Tj. Murchisonce runs on a different line, and does not start 

 from L. costosa, because specimens closely resembling L. Murchisonce have been 

 found in the Opalinum-zone, as I can testify from recent experience. Such speci- 

 mens, then, have produced L. Murchisonce in the zone of that name ; and the latter 

 has produced L. Lucyi in the Concavum-heds. One or other, probably the latter, 

 genetic series is the parent-form of the Hectici, which continue to the Oxford Clay. 



Ludwigia costosa occurs in the Opalinum-zone at Burton-Bradstock Cliff, and is 

 probably very scarce. 



Plate XX, figs. 11 and 12, show a small specimen in my collection. The 

 example figured by Quenstedt is considerably larger. 



