14 PROCEEDINGS OF THE ROYAL SOCIETY OF QUEENSLAND. 
pertinent to remark that the marine Aptian beds (Roma Series) of 
Queensland contain an assembly of Trigonias, mostly undescribed, but 
none of the genera of this faunule. 
7. crassitesta, 7. inconstans and 7. v-scripta all have a gerontic 
stage in which the V-ribbing ceases and purely concentric rugae succeed. 
This, of course, is not peculiar to Iotrigonia. Also, as in other Trigonias, 
in the gerontic stage, the marginal carina and the area tend to smoothen. 
In the nepionic stage the ribbing is concentric; and an intensification 
of gerontic tendencies with progressive limitation of the normal adult 
Y-ribbing would lead to simple, concentrically ornamented forms. Such 
purely concentrically marked genera as Indotrigonia , Sphenotrigonia 
and Butitrigonia are no doubt end-points of such a tendency in more than 
one lineage of Trigonias. 
Iotrigonia limatula sp. nov. PI. I, figs, la, b. 
Diagnosis: Sharply triangular Iotrigonia with very regular 
V-shaped ribs meeting at an acute angle along a straight axis and with 
a sharp marginal carina and narrow cardinal area. 
Description: Shell thin, evenly but 
only slightly inflated, considerably longer 
than high. Outline sharply triangular, 
modified by a Avell rounded antero-ventral 
angle and a slight rounding of the acute 
postero-ventral corner. The umbo, which 
is very sharply defined and incurved, is 
slightly anterior to the median line (at 
almost one-third of the transverse diam- 
eter). Marginal carina very sharp in the 
earlier growth stages (to 16 mm.), abruptly 
bounding a very narrow lunate and slightly 
concave cardinal area. 
On the disc the first few costae are concentric, gradually assuming 
a Y-shape. Then they become V-shaped, the two branches forming a 
rib converging at a very acute angle. Occasionally an anterior rib 
ceases at the V-axis and is not paired on the posterior side. 
The axis of the V’s is remarkably straight and at right angles to 
the major axis of the shell. Costae are broadly rounded and spaced 
with narrower sulci. The posterior region being somewhat wider than 
the anterior, the costae become wider in that direction and cease just 
before the posterior angle. In the first 16 mm. of the marginal carina, 
where it is very sharp, each costa of the disc reappears as a costa of 
the area, extending slightly obliquely to the cardinal margin. Then, 
when the marginal carina broadens, the area becomes smooth and develops 
a faint longitudinal sulcus. The escutcheon is so narrow as to be 
obsolescent. Both disc and area are marked also with faint growth 
striae. 
Internal structures not known. 
Dimensions of holotype: Length: 36 mm. Height: 24 mm. 
Thickness: 7 mm. (Single valve.) 
Along the main axis the relationship of the region in front of the 
Y-axis to that of the region behind it is precisely 1 : 2, 
