Tertiary.) PALZONTOLOGY OF VICTORIA. { Mollusca. 
the character indicated in the specific name, 7. e., the remarkable 
compression of the ribs into acute angular ridges; and from the 
same cause the spinous tubercles do not form the broad, blunt, 
transverse tubercles which they do in the recent species, in which 
latter the ridges form broad, obtusely flattened, almost square ribs 
when viewed from the margin in a position in which those of the 
present species form a series of acute angles. 
Not uncommon in the Older Pliocene beds of Mordialloe, in 
Hobson’s Bay. Rare in the Upper Miocene beds of Muddy Creek. 
EXPLANATION OF FIGURES. 
Plate XIX.—Fig. 1, left valve, natural size. Fig. 1a, portion of surface magnified to show 
the acute ribs and tubercles. Fig. 16, dotted outline of profile to show the depth. Fig. 2, right 
valve. Fig. 3, portion of surface of the most nearly allied recent species, the T. Lamarckhi, to 
show the broad, blunt, flattened tubercles and ridges, contrasting with fig. la. 
Prats XIX., Fias. 4, 5. 
TRIGONIA SEMI-UNDULATA (McCoy). 
Descriprion.—Rotundato-oblong, little longer than deep, moderately convex ; 
anterior and ventral margins broadly rounded; posterior margin nearly straight, 
abruptly truncated, forming an angle of 120° with the hinge-line; posterior slope 
flattened and radiated with about 10 or 11 strong obtusely rounded ridges, separated 
by rather wider flatter spaces, and crossed by lines of growth near the margin, closer 
and spinulose near the beak, and followed on the lunule close to the hinge-line by 
6 or 7 much smaller spinulose ridges; middle and anterior portion of the valve 
covered with narrow rounded slightly undulating ridges, nearly parallel with the 
ventral margin, crossed, except on the anterior portion, by rather faint impressed 
sulci radiating from the beak to the ventral margin, nearly the same distance apart 
as the ridges of the posterior slope. Length of largest specimen, 2 inches 4 lines ; 
propor can width from beak to ventral margin, 8°, ; length of anterior side, +29, ; 
ength of hinge-line, =%°,; of truncated posterior margin, ,%°,; depth of one valve, 
ao 3 average length, 14 inches. 
REFERENCE.— TZ semi-undulata (McCoy), Geol. Mag., v. 3, p. 481. 
I sent labelled specimens of this species to the second Great 
Exhibition in London. Mr. Jenkins gives a figure of it in the 
Quarlerly Journal of Science, in which the transverse ridges are 
not sufficiently thin, numerous, or undulated. 
Easily distinguished from any known recent or Tertiary species 
by the rippled appearance produced by the undulated concentric 
ridging of the anterior two-thirds of the valves ; the posterior slope 
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