34 Transactions of the Boyal Society of SontJi Africa. 
Observations. — The False Bay deposits have yielded some excellent 
examples of this form of Trigonarca, illustra.tmg three stages of growth. 
The largest is a right valve with an imperfect postero-ventral end, 
and adherent to its upper surface and partly covering the carina- 
tion is the lower valve of an Exogyra. It is of very similar size to 
Orbigny's type specimen figured on plate 317, figs. 1 and 2, and appears 
to have the same amount of arching, an equally wide posterior area, 
and a ligamental region of corresponding dimensions. The dentition 
of this valve is, however, somewhat coarse, whilst the antero- 
terminal denticles are more horizontal than oblique, as characterise 
the other specimens with which this is associated. As representing 
the more normal size of the species are three right valves, the 
smallest agreeing with fig. 3 of the original plate, the others being 
of relatively larger proportions. The younger stage of the shell is 
exemplified in the three remaining valves (two right and one left), 
which show all the characteristics of the others, but which on 
account of their much smaller size appear to be somewhat shallow, 
although in reality their measurements are quite proportionate. 
Dimensions (of five single valves). — 
Large. Medium. Small. 
Length (about) 65 50 to 59 30 to 40 mm. 
Height.. 45 30 to 39 20 to 25 „ 
Diameter 25 16 to 20 7 to 10 ,, 
These valves, although with occasional fractured margins and 
with surface structures often replaced by a formation of radial 
calcite both internally and externally, are singularly well preserved, 
besides being entirely free from matrix, so that the interiors are 
properly exposed. The teeth show serrated summits, especially in 
old forms, and minutely plicated lateral surfaces ; the adductor scar 
markings are well developed, together with the prominently project- 
ing curved lamina, which circumscribes the base of the posterior 
impression ; a series of rather wide, smooth plications, separated by 
narrow grooves, occur just above the pallial line in radial arrange- 
ment, being strongest in the region of the postero-ventral angle. 
The external sculpture of the valves includes not only the concentric 
growth-lines, which are periodically more strongly marked than the 
others, but also a radially striated ornamentation is present, more or 
less obscure in aged specimens though well defined in youth. This 
radial sculpture is usually restricted to the end regions of the valves, 
but in the largest of the present specimens it can be traced over 
nearly the whole anterior face of the valve. It is in connection with 
this radial ornament that the present Trigonarca differs from the 
true ligeriensis, which is a shell w^ithout evidence of radial orna- 
