316 ~~ Prof. F. M‘Coy on a new Species of Trigonia. 
XL.—On a third new Tertiary Species of Trigonia. By 
FrepericK M‘Coy, Professor of Natural Science in the 
University of Melbourne. 
(Plate XVIII. B.] 
To the Editors of the Annals and Magazine of Natural History. 
GENTLEMEN, 
The genus 7r¢gonia has furnished an extraordinary apparent 
exception to the usual distribution of genera in time, according 
to which a genus living in the older periods of the world’s 
history, and becoming extinct during a subsequent geological 
period, is not found to reappear at a still more recent epoch. 
Trigonia abounding in the whole of the Mesozoic periods from 
the Lias to the Chalk, represented by many species, seemed 
suddenly to become extinct with the commencement of the 
Eocene Tertiary period, and, being absent in all known Ter- 
tiary formations, seemed to reappear in the present seas of 
Australia; and as none of the well-searched Tertiary deposits 
of Europe or America showed any trace of such shells, a well- 
detined case of exception to the above-mentioned rule seemed 
established, until some years ago I described two species, 
distinct from the living ones, found in the Tertiary formations 
near Melbourne with Aturta, Carcharodon angustidens and 
C. megalodon, Otodus Desori, Oxyrhina trigonodon, Squalo- 
don (Phocodon), and other clearly characteristic Tertiary as 
distinguished from modern types. 
As therefore the announcement of the fact will probably be 
of interest both to zoologists and geologists, I beg to forward 
you a figure and description of a third Tertiary species of the 
genus, which I have lately recognized amongst some speci- 
mens sent to me, as Paleontologist of the Victorian Geological 
Survey, from the eastern portion of the colony, the district 
of Gippsland, of which hitherto comparatively little was 
known. 
Trigonia Howitti (M‘Coy). 
Spec. char. Rotundate rhombic; substance of shell thick ; 
tumid towards the beak, anterior side rounded, posterior slope 
moderately flattened in two planes divided by a very obtuse 
angle marking the margin ; ventral margin moderately convex, 
posterior edge nearly at right angles to the ventral edge, slightly 
rounded in respiratory portion, forming an angle of about 150° 
with hinge-line in anal portion ; about four narrow quadrate 
radiating ridges on each division of the posterior slope, sharply 
separated by deep flattened spaces equal to about their own 
width ; about fourteen thick, prominent, rounded radiating 
