LIFE ON THE EARTH. 95 



freshwater group, Unionidse, belong nearly all the 

 equivalved bivalves of the Lower Palaeozoic, and the 

 larger portion of those found in the Upper Palaeozoic 

 formations. They are numerous in the later strata, 

 and excepting Trigoniadae still remain so. The larger 

 group of Siphonida begins to be plentiful in the 

 Oolitic and Cretaceous rocks, and is much more 

 abundant in the tertiary strata and existing oceans. 



Two conspicuous fossil genera, Trigonia and Pho- 

 ladomya, are represented by one living species to 

 each. The former is confined in a fossil state to 

 Mesozoic Strata, and in a recent state to the Austra- 

 lian shore ; the latter, a constant companion in 

 Mesozoic Strata, is found also in the Eocene beds, 

 and has been discovered living off the Island of 

 Tortosa. The recent species is in each case judged 

 to be distinct from the fossils. What strong affinities, 

 however, obtain in each case between the fossil and 

 the living races will appear by attending to the form, 

 surface ornament, commissure of valves, hinge, um- 

 bones, and muscular impressions. 



Pholadomya in every age preserves a striking con- 

 formity of characters radiating ribs on a part of 

 the surfaces, usually swollen at intervals by pro- 

 minent laminae of growth, a thin tumid oblong 

 shell, prominent beaks, gaping posterior end, ob- 

 scure hinge-teeth. The recent species differs but 



