182 
CONCHIFERA. 
Trigonia. 
ed, divergent, two in the right valve transversely grooved on 
both sides; the grooves regularly marked, each forming the 
segment of a circle ; four teeth in the left valve grooved in one 
side only, but these alternately in pairs ; consequently the four- 
teeth of this valve receive within their grooved sides the two 
teeth of tho right valve ; two principal muscular impressions, 
the lateral ones very distinct, one of which is situate close to 
the superior end of the cardinal tooth, and a little behind it; 
the other somewhat more distant, with a minute one between 
it and the cardinal tooth ; pallial impressions almost entire; 
ligament marginal, thick, rather short, and external. 
1. Tkigonia costata.— The Ribbed Trigonia, pi. LXXV. 
fig. 4. 
F. costata . Parkinson, Org. Rem. III. pi. 12, fig. 4. 
Sowerby, I. p. 195, pi. 85. 
Trigonal, posterior side somewhat ventricose, with a series 
of transverse, elevated, curved, smooth ribs, and smooth inter¬ 
costal spaces ; anterior side large, divided from the flattened 
anterior by an elevated, longitudinal, obliquely curved, creu- 
ated rib, reaching from tho beak to the margin, and projecting 
a little beyond it in some specimens ; two additional equi¬ 
distant longitudinal ribs divide the side ; several longitudinal, 
raised, crenated, thread -like stria? occupy the intercostal spaces; 
beaks nearly central, and sub-acute. 
A widely diffused species, existing through various forma¬ 
tions, viz.: the Oxford Clay, Osmiugton, Dorsetshire; tho 
Coral Rag, Malton and Steeple Ashton; the Great Oolite, 
White Nab, Stoneficld; and the Inferior Oolite, Limplcy 
Stoke and Cotswald Hill. 
2. Tkigonia elongata. —The Elongated Trigonia, pi. 
LXXII.* fig. 13. 
T. elongata . Sowerby, V. p. 39, pi. 431. 
Elongated; sub-triangular, slightly oblique, and gibbose ; 
beaks nearly central, a little reflected ; anterior side moderately 
arcuated, with a series of transverse, elevated, smooth ribs, 
with wide intervening furrows; posterior side separated from 
tho anterior side by a longitudinal curved furrow and a crenu- 
lated rib, with two additional distant ribs; the broad inter¬ 
vening spaces being covered with wide-set, longitudinally 
curved stria?, crossed by waved transverse stria?, producing a 
scaly appcaranco. 
In the Lower Greensand, Ashford, Kent, and tho Portland 
Stone, Dorsetshire. 
3. Trigonia duplicata. —The Two-plaited Trigonia, pi. 
LXXII.* fig. 14. 
T. duplicata. Sowerby, III. p. 03, pi. 237, figs. 4, 5. 
Transversely and obliquely oblong ; somewhat compressed ; 
anterior side large, with small, arcuated, curved ridges ; and a 
strong tuberculated curved rib separating it from the posterior 
side, which is small, and covered with obliquely transverse 
narrow furrows; beaks rather large and prominent; basal 
margin’crenated. 
The Inferior Oolite, Glaizedale, Yorkshire. 
4. Trigonia pennata. —Tho Winged Trigonia, pi. LXXII.* 
fig. 9. 
T. pennata. Sowerby, III. p. G4, pi. 237, fig. 0. 
Obliquely oblong, and considerably arcuated; tho anterior 
side arched, with from ten to twelve elevated, obliquely curved 
ribs, which are striated on their fronts ; with smooth inter¬ 
costal spaces; posterior side flattened, concave, and divided 
into two lobe 3 by a longitudinal furrow, on each side of which 
a series of tuberculated ribs diverge downwards; beaks sub¬ 
acute, a little bent; basal line cleft. 
The Greensand, Teignmouth, Devonshire. 
5 . Trigonia striata. —The Striated Trigonia, pi. LXXII * 
fig. 15. 
T. striata. Sowerby, III. p. 03, pi. 237, figs. 1,2, 3. 
Elongated, sub-triangular, inflated; posterior side with large, 
thick, crenated, transverse ribs; auterior side with numerous 
oblique, waved stria?, subdivided by a longitudinal furrow; 
basal line of the posterior side waved ; beaks sharp-pointed ; 
beneath them a longitudinal, lanceolate lunule. 
The Inferior Oolite, Dundry, Yorkshire, and Cotswold 
Hill. 
0. Trigonia angulata.— The Angular Trigonia, pi. LXXV. 
fig. 1. 
T. angulata. Sowerby, VI. p. 9, pi. 508, fig. 1. Ib. T. 
clacellata , var. I. p. 197, pi. 87, lower figs. 
Transversely sub- triangular and elongated ; rather convex; 
anterior side sub-arcuated, and abruptly sloping ; surface with 
transverse, curved, raised ribs, each of which, on reaching the 
disk, changes into a continuous series of nodules; posterior 
side much produced, and divided from the anterior by a curved, 
longitudinal, crenated rib, with the extremity considerably 
acuminate and truncated; crossed by transverse, wide-set 
stria?; the upper edge concave. 
The Inferior Oolite, Nunney and Blue Wick. 
7. Trigonia al^for.mis.— Tho Wing-shaped Trigonia, pi. 
LXXV. fig. 7. 
T. alcvformis. Parkinson, Org. Rem. III. p. 170, pi. 12, 
fig. 9. Sowerby, III. p. 27, pi. 215, figs. 1, 2, 3, 4. 
Sub-triangular; wing-shaped; anterior side arcuated, and 
occupying tho greater part of tho valve; with many trans¬ 
verse, tuberculous ribs, which becomo smooth, plain, and nar¬ 
row towards the inner side ; intercostal spaces smooth ; pos¬ 
terior side lengthened into an acuminated, sub-truncated beak, 
and separated from tho other portion by a broad, curved, flat¬ 
tened ridge, extending from tho beak, and rapidly widening as 
it approaches the basal margin ; beyond the ridge is a flat 
space, crossod by oblique, flat ribs, divided by very narrow 
furrows; beaks acute, and much incurved. 
In tho Gault, Vale of Wardour; the Lowor Greensand, 
Broughton, Blackdown, and Lyme Regis. 
8. Trigonia imbricata.— The Imbricated Trigonia, pi. 
LXXV. fig. 2. 
T. imbricata . Sowerby, VI. p. , pi. 507, figs. 2, 3. 
Triangular ; compressed ; anterior side with fivo or six trans¬ 
verse, imbricated ribs ; posterior sido obliquely truncated, with 
oblique, transverse, flattened ribs. 
The ribs upon this shell resemble a series of terraces, situate one 
above another. 
The Great Oolite, Ancliff. 
9 . Trigonia quadrata.— The Squarish Trigonia, pi. 
LXXV. fig. 5. 
T. quadrata. Sowerby, Geo. Trans. 2d Ser. IV. p. 342, pi. 
17, fig. 12. 
Sub-quadrangular; compressed; anterior sido short and 
much rounded; posterior sido broad aud truncated; beaks ob- 
