TBIGONIA. 155 



are at present known. These molluscs, wliicli inhabit 

 deep -water on rocky bottoms, must therefore be extremely 

 rare, although from the nature of their habitat it is 

 probable that uot so many specimens are obtained as 

 might otherwise be the ease. In the Tertiary period only 

 eleven species are known, of which two are from the 

 Pleistocene, two from the Miocene, and seven from the 

 Eocene. Directly, however, we reach the Cretaceous, the 

 number of species suddenly leaps up to 208, while the 

 total number of Secondary and Palaeozoic species is upwards 

 of 11-15. Accordingly, out of a total of 1160 representa- 

 tives of the genus, only fifteen are of post-cretaceous age, 

 of which but four now exist, and these ajjparently poorly 

 represented in individuals. Here, then, we have indeed a 

 striking instance of a " living fossil." 



The well-known bivalve shells named Trigonia afford a 

 scarcely less well-marked case of the persistence of early 

 types. This genus was originalljr named in 1791, on the 

 evidence of an extinct sj^ecies, but when fully described 

 by Lamarck in 1804 some few recent living examples had 

 also been obtained. Por the benefit of those of our readers 

 who may not be familiar with these molluscs, it may be 

 mentioned that the living Triyonu'p are rather small shells, 

 of about an inch and a half in diameter, characterized by 

 their somewhat triangular shape, and the strongly-marked 

 transverse ribs, marked with rough tubercles on the outer 

 surface. Internally the shell has a polychroic pearly 

 lustre ; while the peculiarly- shaped and striated inter- 

 locking hinge, when once seen, can never be mistaken. At 

 the present day the TrigoniR' are represented only by some 

 five closely allied species or varieties, confined to the 

 Australian seas ; while in the Tertiary, although more 

 widely distributed, they were likewise rare. In the Secon- 

 dary period, where they range down to the lias, these 

 shells were, however, extremely abundant, and attained 

 far larger dimensions than their existing relatives. Indeed, 

 in the oolites Trigoniie were some of the most common 

 molluscs, whole slabs of rock being sometimes found 

 paved with their handsomely sculptured shells, while all 

 who have visited the Isle of Portland must be familiar 



