Tertiary.-] ' PALiEONTOLOGY OF VICTORIA. [MoUusca. 



Plate XXVII., Figs. 1-2c. 



TRIGONIA HOWITTI (McCoy). 



[Genus TRIGONIA (Bbdg.). (Sub-kingd. MoUusca. Class Dithyra. Order Pectinacea. 

 Fam. Trigouiidas.) 



Gen. Char. — SAe// equivalye, inequilateral, subtrigonal ; outside ridged, inside pearly ; riglit 

 Talve with a V-shaped pair of large diverging teeth, transversely sulcated on each side, received 

 between 4 teeth similarly sulcated on one side, in the left valve,] 



Description. — 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 marg-in moderately 

 convex, posterior edg-e nearly quadrangular, witii ventral edge slightly rounded in 

 respiratory portion, forming an angle of about 150°, with hinge line in anal portion, 

 about 4 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 tliick, prominent, rounded, radiating ridges from the beak to the ventral 

 margin, separated by slightly narrower deep concave spaces; near the beak (for about 

 \ an inch) all the ribs set with strong, blunt, transverse tubercles, about their own 

 thickness apart (about 5 in 3 lines), but on the adults the middle and lower ends 

 of the ribs are marked with irregular lines of growth, like the intervening hollows, 

 except the 7 or 8 anterior ones, on which the large blunt tuberculation is continued 

 to tlie ventral margin (about 3 in 2 lines); length from anterior to posterior end 2 

 inches 3 lines; proportional width from beak to opposite margin, j%%; depth of one 

 valve, ^5%; hinge line, fj^^. 



This species is much larger, thicker, and stronger than the living 

 or the other two Tertiary species, and is readily distinguished by 

 the tuberculation (except near the beak) being confined to the 

 anterior ribs, leaving the middle and posterior ones only slightly 

 ■R^-iukled by lines of growth. The inner edge is strongly toothed 

 by the projecting ends of the channels between the radiating ribs. 

 Sometimes the two small most posterior ridges bear tubercles. 



This species was collected by Mr. Howitt from the Ijeds of 

 sandy marl at Jemmy's Point, near the entrance of the Gippsland 

 Lakes, containing Struthiolaria and other forms which I have 

 observed in the Pliocene Tertiaries of New Zealand, but not of any 

 other locality in Victoria. I have great pleasure in naming so 

 interesting a fossil after so excellent and zealous a geologist as 

 Mr. Howitt has proved himself in the Gippsland District. 



Explanation of Figures. 

 Plate XXVII, — Fig, 1„ exterior of left valve, natural size. Fig. la, interior of ditto. 

 Fig, 2, exterior of right v.alve, natural size. Fig, 2a, interior of ditto. Fig, 26, magnified 

 distinct tubercles of anterior ribs. Fig. 2e, magnified more posterior ribs, with obsolete 

 tubercles, 



[ 31 ] 



