LUDWIGIA RUDIS. 103 



Ludwigia rudis, 8. Buchman. Plate XV, figs. 11 — 17; Plate A, fig. 7. 



Discoidal, somewhat compressed, subcarinate ; whorls subconvex, ornamented 

 with coarse, rounded, reflexed, bifurcating ribs, the ends of which slightly project 

 forwards on the ventral area. This area, scarcely defined, is smooth and sloping, 

 and carries a small but distinct carina. The inner margin is scarcely defined, 

 except on the last whorl, when it is sloping and slightly concave. The inclusion is, 

 in young specimens, about a half, and in older two-thirds, of the preceding whorl. 

 The umbilicus is open and somewhat shallow. The convex portions of the whorls 

 exhibited therein, and the coarse ribs which they carry, give a somewhat tumid and 

 knotted appearance to the exposed parts of the umbilical coil. The mouth-border 

 has a lateral process or horn ; and on the ventral area it is pointed and slightly 

 projected. The ribs continue to be well marked right up to the mouth-border. 



This species, having every characteristic of the genus Ludwigia, is evidently 

 descended from L. costosa, and is therefore a close relation of L. comu. It differs 

 from L. costosa by having a somewhat larger umbilicus, more numerous ribs, and 

 a less distinct carina, besides being somewhat thicker. In the character of its 

 coarse ornamentation this species has some resemblance to certain coarsely-ribbed 

 forms of L. Murchisonce ; but it may be distinguished from them by possessing the 

 compressed whorls which appertain only to the more finely ribbed varieties of that 

 species. The more compressed forms (PL XV, fig. 13) approach L. comu, but may 

 be separated by their coarser ornamentation, thicker, more convex whorls, larger 

 umbilicus, and less prominent carina. 1 From Lioceras apertum, to which it exhibits 

 an extraordinary convergence, this species is separable by the same characters which 

 separate the genera Ludwigia and Lioceras, namely, the ribs frequently bifurcate, 

 sharply reflexed, and conspicuous on the inner area ; the sutures set further apart 

 from one another, simpler, with smaller accessory lobes, fewer auxiliaries, and greater 

 difference in size between superior and inferior lateral lobes. Besides this, the ends 

 of the ribs are bent forwards on the ventral area in Lud. rudis and not in Lioc. 

 apertum ; the carina is more prominent ; the whorls are more convex ; and the 

 umbilical coil is more tumid and more knotted. 



In Dorset Ludwigia rudis sometimes occurs at Bradford Abbas ; but its chief 

 locality is at Louse-Hill Quarry, near Halfway House. Its horizon is in the 

 Concavum-heds. In Somerset the species occurs at Dundry Hill, near Bristol, 

 where it has been obtained by Mr. B. "Wilson, F.G.S., who kindly sent me 

 his specimens to examine. Nowhere does it seem to occur of large size 

 (the specimen figured in Plate XV, fig. 13, being the largest with which I am 

 acquainted), and it thus seems to form with L. comu and L. costosa a series of dwarf 

 1 The carina of L. comu, PL IV, fig. 2, is not acute enough ; see p. 20. 



