202 Jenkins — On an Australian Tngonia. 



so that no doubt can be raised as to tbeir validity, it was with 

 much interest that I sought to ascertain the exact place in the 

 Tertiary series of Australia of a species having so decidedly a 

 Jurassic facies as this Trigonia semmndidata. The information at 

 my command does not, however, enable me to assign the fossil to its 

 exact position in the series, as the specimens I received were 

 obtained from the detached out-crops of Tertiary beds in the neigh- 

 bourhood of Geelong and Port Philip Bay. But in searching for 

 this evidence I was astonished to find amongst the collection of 

 fossils from the same set of &e(ls— though not from the same stratum 

 as that containing Trigonia semiundulata — two specimens of Trigonia 

 undistinguishable by characters of specific value, from the recent 

 T. Lamarckii now inhabiting the Australian seas. Although, possibly, 

 naturalists who consider minute difi"erences of degree, or an ex- 

 tended range in time, suificient for the foundation of a new species, 

 may be able to separate this fossil from Trigonia Lamarckii, yet to 

 the pal83ontologist the value of the discovery will remain unchanged ; 

 for while the only Tertiary Trigonia hitherto known belongs to a 

 group, differing in every essential character, not generic, from the 

 recent species, we have now evidence of another Trigonia having 

 lived within the limits of the same Geological period as the former, 

 which is absolutely identical with a species which now inhabits the 

 Australian seas. 



The species of Trigonia described and figured by Mr. Lovell 

 Eeeve in his Conchologia Iconica are as follows : — ■ 



1. Trigonia Lamarckii (T. Jukesii, Adams) ; Australia, 



2. ,, uniophora, Gray ; Cape York, Australia. 



3. „ mar garitacea {T.'pectinata, Jjd^-m.) Tasmania. 



4. „ Strangei, Kdid^m.^; Sydney, N.S.W. 



In general character, especially in the size, shape, number, and 

 ornamentation of the ribs, and obtuse truncation of the posterior 

 extremity, the fossil approaches most nearly to T. Lamarckii, the 

 first-named species or variety, which is also the most abundant. On the 

 other hand, it undoubtedly presents some differences, being slightly 

 flatter, rather longer in proportion to its breadth, and somewhat 

 more inequilateral. The fossils are also all small specimens ; but it 

 cannot yet be certainly stated that, in Tertiary times, no individuals 

 of the species attained the size of adult recent examples. With 

 every disposition to make the most of the differences enumerated, I 

 cannot regard them as of even varietal value ; they seem, in fact, to be 

 merely such slight individual peculiarities as can be traced in the 

 members of almost every species, added to a certain caste which, 

 probably, often distinguishes those which inhabit one locality from 

 the population of another. Indeed, the same kind of slight varia- 

 tion may be noticed in the specimens from the two localities which 

 have yielded the species in the fossil state, namely, near Sherbnook 

 Eiver,^ and Mordialloc,^ both in the Colony of Victoria. 



1 Mapped as Miocene by Mr. Selwyn. ^ Mapped as'Older Pliocene by Mr. Selwyn. 



