404 
MOLLUSCA. TRIGONIAD^. Trigonia. 
TRIGONIAD^.. 
Gen. trigonia. — E quivalve, unequilateral trigonal; on 
the right valve are two oblong, flat, diverging hinge teeth, 
transversely grooved on each side ; on the left valve four 
flat hinge teeth, transversely grooved on one side only, dis- 
posed in pairs ; ligament external, marginal. 
1. T. costata . — ^Triangular, with transverse, smooth ribs ; anterior side 
marked Avith many small, and three large, prominent, longitudinal, erenula- 
ted ridges — Curvirostra rugosa, Luid. Lith. 36, t. ix. f. 714 T. cost. Park. 
Org. Rem. iii. 175, t. xii. f. 4:.-— Sower. Min. Conch, t. Ixxxv. — Inferior Oolite. 
2. T. spinosa — Roundish, depressed, many ribbed ; anterior side trunca- 
ted ; ribs oblique, diverging both ways from the ridge that separates the an- 
terior side ; set with short spines. — Park. Org. Rem. iii. 177* t. xii. f. 7* — 
Sower. Min. Conch, t. Ixxxvi. — In Green Sand. 
3. T. -Triangular, rather Avider than long, with ten or more ob- 
lique rows of tubercles ; anterior side straight, with three longitudinal knot- 
ted ridges — Luid. Lith. 36, t. ix. f. IO 7 . Park. Org. Rem. iii. 175. t. xii. 
f. 3. Sower. Min. Conch, t. Ixxxvii. — Oolitic Series. 
4. T. dcedalea. — Obtusely rhomboidal, Avith many oblique roAvs of tubercles ; 
anterior side angular, Avith many scattered tubercles, and one longitudinal 
row bounded by a longitudinal ridge, on which is another row of large tu.< 
bercles — Parks Org. Rem. iii. 176, t. xii. f. 6. Sower. Min. Conch, t. Ixxxviii. 
— Green Sand. 
5. T. eccentrica — Transversely ovate, convex ; anterior side produced, 
posterior side round, with oblique undulating ridges on the surface. — Park. 
Org. Rem. iii. 175, t. xii. f. 5. — Sower. Min. Conch, t. ccviii. f. 1, 2 — In 
G^'een Sand. 
6. T. affinis . — Transverse, oval, gibbose, half covered by transverse ridges. 
— Sower. Min. Conch, t. ccviii. f. 3 — Green Sand. 
7 . T. 'VVing-formed ; anterior side produced, truncated, Avith a 
broad compressed ridge extended to the beak ; posterior side costated, round- 
ed ; costae many, thick, irregularly crenulated, oblique, recurved, and atte- 
nuated towards the ridge on the anterior side, Avhere they terminate — Park. 
Org. Rem. iii. 176, t. xii. f. 9. Sower. Min. Conch, t. ccxv.—In Green Sand. 
8. T. gihhosa Transversely oblong, gibbose, slightly arched, concentrical- 
ly undulated, or irregularly sulcated ; anterior side separated by a broad 
longitudinal furrow or plane space — Sower. Min. Conch, t. ccxxxv. ccxxxvi. 
— Upper Oolite. 
9. T. s#na#a.--Obtusely triangular, convex ; with arched, oblique, crenu- 
lated costae upon the middle and posterior side ; anterior side nearly covered 
by numerous, elevated, sharp ridges. — Sower. Min. Conch, t. ccxxxvii. f. 1, 2, 
3, — Inferior Oolite. 
10. T. Transversely oblong ; anterior side marked with small 
undulating ridges, and bounded by a strong tuberculated ridge ; over the 
middle are longitudinal crenulated costae, suddenly reflected over the poste- 
rior side ; on the front are short intermediate ridges. — Sower. Min. Conch, 
t- ccxxxvii. f. 4, ^.—Inferior Oolite. 
