TRIGONIID^. 245 



BARTLETTfA, H. Adams, 1866. 



Distr. — B. StefanensU^ Moric cxxiv, t). Amazon River. 



Shell free, equivalve, closed, inequilateral, the anterior por- 

 tion being peculiarly produced and rugose, the ventral edge 

 insinuated, hinge edentulous; ligament marginal, partially 

 internal, supported by strong fulcra, muscular scars two, mar- 

 ginal ; pallial line entire. 



This singular shell has the irregular growth of the attached 

 genera, except that it is equivalve ; in possessing two muscular 

 scars it is nearest allied to ^theria. 



( Trigoniacea ) 



Family TRIG0NIID_^. 



Shell equivalve, close, trigonal, with the umbones directed 

 posteriori}' ; ligament external ; interior nacreous ; hinge-teeth 

 few, diverging ; pallial line simple. 



Animal with the mantle open ; foot long and bent ; gills two 

 on each side, recumbent ; palpi simple. 



Trigonia, Bruguiere. 



Etym. — Trigonos, three-angled. 



Syn. — Lyriodon, G. Sowerby. Myophorella, Bayle. 



Dist7\ — 3 sp. (or varieties?). Australia. Fossil, 100 sp. 

 Devonian — ; Europe, United States, Chili, Algeria, Cape, 

 South India. T. pectinata, Lam. (cxxv, 18). 



Shell thick, tuberculated, or ornamented with radiating or 

 concentric ribs ; posterior side angular ; ligament small and 

 prominent ; hinge-teeth 2"3, diverging, transversely striated ; 

 centre tooth of left valve divided ; pedal impressions in front of 

 the posterior adductor, and one in the umbo of the left valve ; 

 anterior adductor impression close to the umbo. 



Animal with a long and pointed foot, bent sharply, heel promi- 

 nent, sole bordered by two crenulated ridges ; palpi small and 

 pointed ; gills ample, the outer smallest, united behind the bod}^ 

 to each other and to the mantle. 



The shell of Trigonia is almost entirely nacreous, and usually 

 wanting or metamorphic in limestone strata ; casts of the inte- 

 rior are called " horse-heads " by the Portland (England) quarry- 

 men ; thej^ spoil the stone. Silicifled casts have been found at 

 Tisbury, in which the animal itself, with its gills, was preserved. 

 The species with the posterior angle of the shell elongated have 

 a siphonal ridge inside. The epidermal layer of the recent shell 

 consists of nucleated cells, forming a beautiful microscopic 

 object. A Trigonia placed by Mr. S. Stutchbury on the gunwale 

 of his boat leapt overboard, clearing a ledge of four inches ; they 



