CONCLUDING SYNOPTICAL OBSERVATIONS. 231 



Stol., and two others of the Scabra, equally insignificant, T. crenifera, Stol., and 

 T. minuta, Stol., complete the list of Indian Cretaceous Trigonige, 



The section Pectinida was established by Agassiz in 1840 (' Trigoniae,' pp. 10 

 and 48) upon the Trigonia pectinata of Lamarck, at that time the only known species 

 of the section, so named from the external resemblance which their ornamentation bears 

 to the Pectines and Limce ; the species, both living and Tertiary, are exclusively Australian. 

 They present in their suborbicular Cardium-like forms, in their crenulated costse radiating 

 from the umbones, in their areas destitute of bounding carinse or of divisional sulcations, 

 in the absence of any clear separation between the dorsal and the anal or siphonal 

 portions of the surface, a remarkable contrast to the Trigonia; of the other continents ; 

 differences which are rendered the more remarkable when we examine the hinge features, 

 which present little or no modification of the older Mesozoic forms of the genus ; even 

 the changes observable in the interiors of certain of the Cretaceous Scabra and Quadrata 

 have also disappeared, and in the Pectinida we find reproduced unchanged the more 

 ancient hinge features of the Mesozoic Glabra in all their original prominence. 



The European and American Tertiary formations, although occupying such extensive 

 tracts of country and presenting every gradation of moUuscan life, from extinct to living 

 forms, are altogether destitute of the genus : it is only in the Tertiary Australian deposits 

 of Victoria and of South Australia, associated with other existent generic forms, that we 

 again discover Trigonice, represented solely by the group of the PectinidcE, and more or 

 less nearly allied to the few forms of the genus which inhabit the seas and tidal waters of 

 the same region. This great hiatus in the chain of Trigonia life may possibly be 

 eventually filled up by discoveries in some unknown series of the Tertiary formations. 

 Widely, indeed, as the Pectinida are separated from the usual Mesozoic sectional forms, we 

 discover some resemblance to a portion of the Cretaceous Scabra both in their surface 

 ornaments, and in the not less important absence of carinre upon the superior and 

 siphonal portions of the shell. There may also be observed in an Australian Tertiary 

 species {T. Ilowitii, McCoy) a tendency to efiacement of the ornaments over the middle 

 portion of the valves, various examples of which occur in the Mesozoic Undulata and 

 Glabra. Perhaps some modification of this statement may be deduced from an 

 examination of some known Cretaceous Trigonia. In T. disparilis, d'Orb. (' Ter. Cret.,' 

 pi. 229), one of the Scabra of the Terrain Senonien, or highest chloritic marls of Tours, 

 some approximation may be seen in the numerous crenulated costae which radiate from 

 the umbones over the middle portion of the valves ; also in the minute T. pulchella, 

 Reuss (Bohm., tab. 41), from a similar position in Bohemia ; if, indeed, the latter 

 species be not the very young condition of the former, which I am inclined to believe. 



The American T. Humboldtii, von Buch, which has likewise costellae radiating from 

 the umbones over the upper, the anal, and the median portions of the shell, may also be 

 regarded as a transitive or connecting species ; these, however, arc rare and exceptional 

 forms in the great section of the Scabra. 



