27 
TRIGONIA alagformis. 
TAB. CCXY. 
Syn. T. aliformis. Parkinson III . 176. t. XII. 
/9. 
Spec. Char. Wing-formed ; anterior side pro- 
duced, truncated, with a broad compressed 
ridge extended to the beak ; posterior side cos- 
tated, rounded, costag many, thick, irregularly 
crenulated, oblique, recurved, and attenuated 
towards the ridge on the anterior side where 
they terminate. 
Upon the upper part of the anterior side, above the 
ridge, is a concave surface ornamented with a few irre- 
gularly crenulated ribs, that extend across the ridge 
near the beak, and join attenuated ends of the other 
costae, but are gradually lost upon the produced part of 
the side. The internal margin has a short groove corres- 
ponding to each of the external ribs, and two canals be- 
neath the flat space upon the produced beak- like side, 
which are very evident in relief upon the casts, see fig. 2. 
A long known shell among the siliceous fossils, often 
so admirably preserved in form in the green sand. The 
individuals represented in figures 1, 8, and 4, are se- 
lected from Mr. Miller’s choice collection : fig. 4 is re- 
markable, as it exhibits the great gibbosity of the young 
subject. Fig. 2 is taken from a cast of the inside, in fer- 
ruginous sand, which occurs in Parkham Park, Sussex ; 
I am indebted to G. A. Mantell, Esq. for it ; it appears 
to be abundant, as many are included in the same mass : 
there are casts of the outsides also, allowing spaces 
