238 ON SOME MESOZOIC FOSSILS FROM CENTRAL AUSTRALIA, 



Shell ovately oblong, produced posteriorly, thick, tumid, very 

 convex, umbones somewhat anterior, not recurved, superior border 

 moderately concave, rather elongate, posterior border curved, 

 slightly truncate at the siphonal margin ; anterior border tumidly 

 produced and rounded, posterior groove wide, conspicuous, shallow, 

 widening rapidly from the umbones to the margin, costa irregular, 

 not prominent, some smaller ones occasionally intercalated, all 

 passing horizontally across the valve, slightly undulating ante- 

 riorly, disappearing on the posterior groove ; siphonal ridge 

 smooth, with three narrow rounded ridges, three rather wide, 

 shallow, grooves between, which become faint as they widen 

 out towards the margin, escutcheon long, ovate, with a flat groove 

 on each side, striae on the lower part, the rest smooth, margins 

 raised ; hinge teeth thick, large, prominent, with two grooves on 

 the upper edge and about 13 close, neat, parallel, lateral grooves. 

 The shell is of considerable thickness and is separated into two 

 layers the outer of which is the thicker. 



This fossil manifestly belongs to that section of the large genus 

 2Vigo?iia, which is classed as Glabrce or smooth. The section is 

 characterised by sub-quadrangular or elongately ovate shells 

 moderately compressed, area not margined or indicated by a 

 distinct groove, sides ornamented with smooth concentric ribs, 

 sometimes extending over the area, or becoming quite obsolete 

 before reaching it. The type is T. longa, Agassiz. 



The group of Glabrce is decidedly Cretaceous, most of the species 

 belong to that period, though one or two species rarely extend into 

 the Jura. 



The above fossil comes nearest in its form to T. excentrica, 

 Park, and T. duncombensis, Lycett, both C-reensand species or 

 Lower Cretaceous in Britain. There is a single elongated species 

 of the Glabrce division, from the Cretaceous rocks of America. It 

 comes from Columbia. Trigonia semiculta, Stol., of the Cretaceous 

 rocks of Pondicherry, is a short globose species, with the trans- 

 verse costa interrupted about the middle of the shell by the usual 

 smooth anti-carinal space, but it is not grooved as in the present 

 species, is short, has not the posterior carina, and has the costa 

 much more regular. 



