232 BRITISH FOSSIL TRIGONIyE. 



Within the last few years the Tertiary deposits of Australia have yielded several forms 

 o{Trir/onia ; these all belong to the section of the Pectinida, more or less allied to living 

 forms. One small one, more especially allied to the species of Sydney Harbour, is named 

 by Jenkins T. Lamarckii (' Geol. Mag.,' vol. iii, 186G, pi. 10, figs 3, 7), a name previously 

 given by Matheron to a Cretaceous species (see p. 138). This Tertiary species is named 

 by McCoy T. acuticosta (' Geol. Mag ,' 1866, p. 481), from the beds of Mordialloc in 

 Hobson's Bay. Other Tertiary species are 1\ semiundulata, McCoy, from Bird Rock 

 Bluff, and T. Howitii, McCoy, from near the entrance to the Gippsland lakes. The latter 

 in size nearly equals som.e of the larger Mesozoic forms, the length is twenty-six lines, 

 the height twenty lines ; it is remarkable for the great length of the hinge-border 

 compared with the short perpendicular siphonal border; the middle portion of the shell 

 has a tendency to the effacement of surface ornaments ('Ann. and Mag. Nat. Hist.,' 

 ser. 4, vol. xv, pi. 18). 



The distinctive differences of T. acuticosta as compared with the species of Sydney 

 consist in the far more numerous costae, their angulated forms, the lesser convexity 

 of the valves, their more inequilateral figure, the greater breadth of the posteal slope, 

 and greater length of the siphonal border : differences of small importance when viewed 

 separately, but in the aggregate appearing to justify the separation claimed by McCoy 

 for the Tertiary forms. They are, however, for the most part such as have been regarded 

 as varieties among the Mesozoic Trigonia figured in this Monograph ; and, should it 

 eventually be decided to separate this small Tertiary species from the neighbouring living 

 forms, it would undoubtedly be necessary to erect into species certain Mesozoic forms 

 here tabulated as varieties. 



T. semiundulata, McCoy, considered to approximate to the Jurassic TrigonicB 

 costatcB in its apparent concentric ribbing, is, nevertheless, one of the Pectinida, having 

 the anteal and mesial costae only slightly defined, crossed by undulating concentric ridges 

 or lines of growth ; in no other feature does it approach to Mesozoic forms. 



The Tertiary Australian Trigonice deposited in the Melbourne National Museum 

 have been assigned to the Miocene and Pliocene stages from a comparison of the 

 percentage of living forms with which they are associated ; but the question whether such 

 a rule is applicable to Australian geology, and whether it affords a criterion whereby it 

 may be measured with European Tertiary deposits, has yet to be determined. 



The foregoing three Tertiary forms are so distinct from each other that there can 

 scarcely remain a doubt of the propriety of regarding them as separate species ; the small 

 T. acuticosta, in its approach to the living Australian forms, will require further investi- 

 gation, which will include the question of the separation of the living PectinidcB into 

 species and varieties, concerning which naturalists are much divided in opinion. Our 

 present very insufficient knowledge upon this subject may be greatly augmented by the 

 results of future dredging operations. A more precise estimate will be thus obtained 

 of the hydrographical Hmits and habitats of forms, at present tabulated as species. 



