16 INFERIOR OOLITE AMMONITES. 



Tribe— ^IGOCEIIATID^I, Neumayr. 



Subteibe— HARPOCERATIN^], Neumayr. 

 Generic group — HAHMATOCERATiDiE, BucJcman. 



Generic subgroup — Hildoceratin^:, Buchnan. 



Genus — Ludwigia, Bayle, 1878. 

 {Type — Ludwigia Mtjechison^;, Sowerly, sp.) 



Definition. — Variable, sometimes much compressed ; strongly marked reflexed 

 bifurcating ribs, primary large, especially conspicuous in youth, and sometimes 

 almost developing spines. Ventral area plain, without ribbing, somewhat flattened, 

 keel variable. Mouth border with an elongated lappet on the lateral area, and very 

 slightly produced on ventral area. Inner margin always concave. Umbilicus open, 

 the inclusion generally half the whorl. Suture lines very simple, not numerous, 

 having a well-defined space between each succeeding suture. Siphonal saddle 

 divided by a somewhat rudimentary accessory lobe, lateral saddle scarcely 

 divided by any such lobe. Inferior lateral lobe considerably smaller than superior 

 lateral. Three small auxiliary lobes. 



Remarks. — This genus does not contain many species, and is hard to define on 

 account of its variability. The most constant characters are the suture lines and 

 the ribs. 1 It will be seen that the suture line 2 enables us to distinguish this genus 

 from Lioceras, being more simple, farther apart from one another, possessing only 

 a small accessory lobe in the siphonal and scarcely one in the lateral saddle, which 

 lobes are always well developed in Lioceras. The suture line in Ludwigia is also 

 less branched, and the saddles proportionately deeper than in Lioceras. The 

 umbilicus in Ludwigia is also proportionately larger and more open than in 

 Lioceras, and does not increase so irregularly when the body-chamber is present. 



The genus Ludwigia was founded by Bayle in 1878 in his ' Explication de la 

 Carte geologique de la France,' and he places in this genus the following Ammo- 

 nites, viz. L. aalensis, costula, opalina, mactra^ exarata, Sinon, and Murchisonce, 



1 It should be especially noticed that, whilst in Ludwigia the secondary ribs are well developed, 

 yet the primary are as large or larger than the secondary. On the other hand Lioceras has, practically, 

 no primary ribs. The secondary ribs unite more or less about the middle of the side and disappear 

 into lines of growth, or at the most very small ribs which decrease in size as they near the urner 

 margin. This of course does not apply to Lioceras opalinum and others, which are merely striated. 



2 See Plate II, fig. 5. 



