31{> I'rot". F. M'Uuy on a iww Species of Trigonia. 



XL. — (hi a third new Tertiary Species of Trigonia. By 

 Frkdkhick M'(J()V, Professor of Natural Science in the 

 University of Melbourne. 



[Plate XVIII. B.] 



To Uie Editors of tlu Anmih and Magazine of Natural History. 



Gentlemen, 



The genus Trigonia lias furnished an extraordinary apparent 

 exception to the usual distribution of genera in time, according 

 to which a genus living in the older ])eriods of the world's 

 history, and b.^coniing 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 Aturia, Carcharodon angustidens and 

 C. megalodojif Otodus Desoriy O.cyrhina trigonodon, Sqiialo- 

 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 Palaeontologist of the Victorian Geological 

 Survey, from the eastern portion of the colony, the district 

 of Gippsland, of which hltlierto 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 nan'ow 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 



