482 M'Coy — On Tertiary Trigonia. 



witli the ventral margin, crossed, except on the anterior portions, 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 longest specimen, two inches ; proportional width 

 from beak to ventral margin ~°^ ; length of anterior side ^^ ; length 

 of hinge line j^ ; of truncated posterior margin ^^ ; depth of one 

 valve j^ ; average length 1^ inches. 



I sent labelled specimens of this species to the second Great 

 Exhibition in London. Mr. Jenkins gives a figure of it in the 

 "Quarterly Journal of Science," in which the transverse ridges are 

 not sufficiently thin, numerous, or undulated. 



It is 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 

 is abruptly marked by the ridges being only radiating. The trans- 

 verse ridging, though common in the Mesozoic Trigonice, is not 

 found in the recent species. 



Very abimdant in the sandy beds of Bird-rock Bluff Ad. 22 and 

 Ad. 24. 



2. Trigonia aciiticostata (M'Coy). 



Syn. T. Lamar cUi, Jenkins, Gteol. Mag., Yol. III. p. 201, Plate 

 X., fig 3-7 (not of Mathn.) 



Sp. Ch. Eotundato rhombic moderately convex; posterior slope 

 flattened ; anterior and ventral margins rounded ; posterior margin 

 obliquely subtruncate nearly straight ; respiratory angle obtusely 

 rounded ; anal angle about 130°; surface radiated with about thirty- 

 two acutely angular ribs, about thirteen of which are on the pos- 

 terior slope ; the intervening spaces seem wider than the ribs from 

 the sides of each rib gradually converging to an acutely angular 

 line closely set with numerous small thorny tubercles (about seven 

 in. three lines, at six lines from the beak) ; intervening spaces 

 coarsely striated and wrinkled at right angles to the length from 

 anterior to posterior end 1 in. three lines ; proportionate width 

 from beak to opposite point of ventral margui ^^ ; depth of one 

 valve ^ ; length of anterior side ~-^ ; length of hinge line ~ ; of 

 truncated posterior margin j^^ 



This species is easily distinguished, even as a fragment, from the T. 

 Lamarchii, T.pectinata, and other recent species, by the character 

 indicated in the specific name, i.e. the remarkable compression of the 

 ribs into acute angular ridges, Woodcut Fig. 1 ; and from the same 

 cause the spinous tubercles do not form the broad, blmit, transverse 

 tubercles which they do in the recent species, in which 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 Mordialloc in Hob- 

 son's Bay. Eare in the Upper Miocene beds of Muddy Creek. 



The specimens above referred to were collected by the Geological 

 Survey, and are deposited in the National Museum of Melbourne. 



