CHAPTER XXIV: THE THREE-ANGLED CLAMS 



Family TRicoNiiD/t 



Shell equivalve, closed, three-angled, with beaks pointing 

 backward; ligament external; hinge teeth few, diverging; interior 

 pearly; pallial line simple; mantle open; foot long, bent; gills, 

 two pairs. Marine. 



Genus TRIGONIA, Brug. 



Characters of the family. The distinctions of this small genus 

 of Australian bivalves are, (i) the brilliantly iridescent nacre that 

 forms almost the whole of the shell substance; and, (2) the most 

 closely interlocking hinge to be found among bivalves. The 

 sheen of the pearl is sometimes golden, sometimes silvery; the 

 ground colour is orange or deep rose-colour, according to the 

 species. Diverging plates and grooves form comb-like hinge 

 teeth before and behind the beaks. The outer surface of the 

 shell is deeply scored with radiating grooves between rounded 

 ridges, bearing nodose projections; the shell margin is crenu- 

 lated; the gill margin frilled. 



There are one hundred fossil species of this genus, found in 

 the Devonian strata, all over the world. The three living species 

 are confined to Australia. 



The peculiar shell ornamentation found only in this genus 

 of living moUusks occurs in fossil species of the fresh water Unios 

 of Pliocene strata. This coincidence, with other structural simil- 

 arities, leads Neumayr to believe that the great family of the Unio- 

 nidae is derived from Trigonia. 



In the limestone quarries of Portland, England, the stone 

 is often spoiled by the presence of "horse heads" — casts of Tri- 

 gonia, silicified nodules which have gradually filled and replaced 

 the shells, which are so largely of pearl that they gradually 

 disintegrate. Casts showing in detail the structure of the soft 

 parts have been found. 



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