115 



The partial or entire disappearance of whole families which 

 have only had a limited area may be accounted for, by a change 

 of climate a change in the masses of land by depression 

 or elevation the formation of desert belts, such as the Sahara 

 an alteration in the direction of oceanic currents or by sub- 

 marine volcanic disturbance, but is not so easily explained when 

 we have to deal with families which at one time occupied exten- 

 sive areas and ranges, like the Trilobita, Brachiopoda, certain 

 Cephalopoda, such as the Ammonities, and other mollusca Tri- 

 gonioe, Pholadomyce which had gained at one period so firm a 

 hold as to threaten predominance. At this zenith of their 

 career the sentence of decline or extermination was irrevocably 

 passed, and with but fe\v exceptions the records of their existence 

 are only revealed when their rocky sepulchre are exposed to 

 view. 



It is remarkable that the living members of this family are 

 only met with in the seas of Australia, a continent where the 

 marsupial representatives of the Jurassic age also find a home, a 

 period synchronous with the fullest development of the family 

 Trigonia. 



CLAVELLAT^. 



TEIGONIA FORMOSA, Lycett, plate i., fig. 1. 



MONO. BEIT. Foss., TKIGONLE, Lycctt, Pal. Soc., p. 35, plate v., figa. 4-6. 

 Geological Journal, Vol. 35, p. 743, 1879. 



Shell ovately trigonal depressed ; umbones elevated, pointed, 

 and recurved, anterior side moderately produced, both it and the 

 lower border elliptically curved ; superior border lengthened and 

 concave ; area rather narrow, flattened, with closely arranged, 

 acute, transverse striations, a faintly marked oblique, mesial 

 furrow, and bounded by two small densely and minutely dentated 

 carinse ; the escutcheon is concave, smooth, and lengthened > 

 the costated portion of the shell has very numerous narrow, 

 oblique, knotted ridges, which are small at the carina, but 



