CONCLUDING SYNOPTICAL OBSERVATIONS. 225 



lines mucli more prominent than in T. lineata ; the posteal slope is destitute of bound- 

 ing carinas. The presence of a gigantic Crioceras at the same locality is also strongly 

 indicative of the Cretaceous rocks. 



It is evident that further explorations and collections of fossils, and more especially of 

 T. lineata, will be required ere the stratigraphical position of the Queensland beds 

 can be determined without ambiguity. 



On passing from the Jurassic to the Cretaceous rocks the genus Trigonia presents 

 some very remarkable changes ; the large and conspicuous section of the TJndulatce is 

 found to have altogether disappeared, neither is there know^n a single continental 

 example of the Clavellata. 



In the Neocomian period generally the great sectional series of Trigomce, con- 

 stituting the ScabrcB and Quadrates, first appear, replacing the lost CostatcB, Clavellata, 

 and Undulata. Varied in species, considerable in numbers, and conspicuous beyond all 

 other sectional forms in their ornaments and in their individualisation, they continue 

 prominent and generally predominating in every considerable assemblage of Conchifera 

 throughout the varied beds of the Cretaceous rocks in whatever country they are 

 discovered, so that a clavellated Trigonia having an ornamented escutcheon becomes an 

 infallible indication of the presence of the Cretaceous rocks. The converse of this statement 

 holds good as to the clavellated species with smooth escutcheons ; they are all 

 Jurassic. The two examples figured in this Monograph, T. ingens. Lye, PI. VIII, figs. 

 1—3, and PI. XXXVI, figs. 5, G ; and F. Keepingi, Lye, Pi. XXXV, figs. 1, 2, are the 

 only known Cretaceous Clavellata. They have been obtained only within the very 

 limited area occupied by the Middle Neocomian foi-mation in Britain. 



It is in the Cretaceous Trigonia also that we find the genus represented by the 

 greatest variety of figure, ranging from the short, suborbicular, or subquadrate examples of 

 the Qnadrata to the subcrescentic attenuated forms of a portion of the Scabra or group 

 of 2\ aliformis, the latter having their siphonal borders so short that the incurrent 

 and excurrent respiratory orifices are brought into near proximity. They differ likewise 

 from all other of the Mesozoic Trigonia in having the lower borders of the valves 

 toothed, a feature which is reproduced in the living Australian section of the Pectinida. 

 These differences of figure are so considerable that when they arc found to apply to the 

 vital organs it can scarcely be supposed that the Mollusca so differently constituted could 

 have had the same habitats in depths of waters, or that their characteristic habits were 

 similar. Nevertheless, throughout the Trigonia-bearing beds of the Cretaceous rocks, from 

 the Middle Neocomian beds even to the highest beds of Chloritic ]\Iarls, we find these 

 forms so dissimilar associated, their valves sometimes together, but more frequently 

 separated, leading to the inference that the dead Testacea were in some instances drifted 

 to the places where they are found in such considerable niuubers. 



The Cretaceous Quadrata also possess a peculiar featiue which must have a 

 connection with the general economy of the Mollusk. The hinge-processes, even in the 



