1-14 



T. excentrica, Park., Chi. Sands. 

 T. gibbosa, Sow., Portland Oolite. 

 T. Manseli, Lye., Portland Oolite. 

 T. Damoniana, De Lor., Portland Oolite. 

 T. tenuitexta, Lye., Portland Oolite. 



The section Scabr<e, as has already been observed, is dis- 

 tinguished from the rest by its departure from the true trigonal 

 f orm, and is more orbicular than any of the other sections. It does 

 not extend beyond the known limits of the Cretaceous beds, and 

 has three representatives in Dorsetshire, namely : 

 T. Vicaryana, Lye., Chi. Sands. 

 T. Meyeri, Lye., Chi. Sands. 

 T. aliformis, Lye., Chi. Sands. 



The section Costata differs from the previous, in the dis- 

 similarity of the valves, both in shape and ornamentation. The 

 sides are furnished with elevated plain costse, and the area 

 separated by two dentated caringe, each valve being divided 

 longitudinally into two nearly equal portions. There are seven 

 Dorsetshire species of this section. 

 T. monilifera, Agassiz., Kim. Clay. 

 T. Meriam, Agassiz., Calc. Grit. 

 T. elongata, Sow., Ox. Clay. 

 T. sculpta, Lye., Inf. Oolite. 

 T. costata, Sow., Inf. Oolite. 

 T. tenuicosta, Lycett, Inf. Oolite. 

 T. letta, Lycett., Inf. Oolite. 



Towards the close of the Cretaceous period the whole family 

 of Trigonia showed symptoms of decline, which reached its termina- 

 tion in Europe during the Cretaceous age ; not a single species of 

 Trigonia has been met with in any of the Tertiary beds, but it is 

 possible some may have been hardy enough to withstand the long 

 strain of depletion, for there are five living species, all of which 

 are found in the bay of Sidney and the seas of Australia. T. 

 Lamarclcii, Gray ; T. margaritacca, Lam. ; T. nolilis, Adams ; T. 

 Strangei, Adams; T. uniophora, Gray. The last Challenger 

 expedition also brought to light a new species from these seas. 



