222 
DESCRIPTION OF THE FOSSILS. 
T. rhomboidea. Ph. PI. XII. fig. 18, 19, 20. Bolland. 
No lateral plaits ; perforation minute. 
T. seminula. Ph. PI. XII. fig. 21, 22, 23. Bolland. 
One lateral plait ; perforation minute. 
Div. 2. — Plaited; generally oblate ; the middle of the front even or 
elevated. 
T. acuminata. Mart. Sow. PI. XII. 4 — 9. Bolland ; Queen’s County ; 
Kildare. 
The varieties are almost innumerable, and a specific character is at present impos- 
sible. (T. acuminata et T. platyloba Sow.) 
Var. 1. Front angular, a. No mesial plaits ; with or without lateral plaits, fig. 
4 ; Young, fig. 5. 
8. Mesial plaits variable ; with or without lateral plaits. 
y. Whole surface sharply plaited. 
2. Front arched. With mesial plaits fig. 
Young of the same fig. 8, 9, 6. 
T. mesogona. Ph. PI. XII. fig. 10, 11, 12. Bolland. 
Tetrahedral, frontal elevation single or bifid ; sides one or two-plaited. It is a 
miniature copy of the first var. of T. acuminata. 
T. reniformis. Sow. PI. XII. fig. 13, 1 4, 1 5. Bolland ; Queen’s County. 
Reniform ; sides without plaits, inflated ; mesial fold extremely wide. «. No 
mesial ridge, p. three to five obtuse mesial ridges, 7. three to five acute mesial ridges. 
T. pugnus. Mart. Sow. PI. XII. fig. 17- 
Oblato-deltoidal ; all the plaits short, rather obtuse ; surface striated. 
T. sulcirostris. Ph. PI. XII. fig. 31, 32. Bolland. 
Rhomboideo-deltoidal, edge sharp, plaits obtuse ; mesial plaits five to nine ; upper 
valve sulcate toward the beak. a. plaits numerous, p. plaits fewer. 
T. flexistria. Ph. PI. XII. fig. 33, 34. Bolland. 
Oblate, depressed, mesial elevation rounded ; lower valve smaller, flatter, with in- 
conspicuous beak ; many obtuse striae, much curved on the sides. 
T. tumida. Ph. PI. XII. fig. 35. Bolland. 
Oblate, tumid, lower valve flatter, with inconspicuous beak ; striae strong and 
rounded on the middle, smaller and curved on the sides. 
T. pleurodon. Ph. PI. XII. «. fig. 25, 26, 28. /3. 16, 29, 30. y. 27. 
Bolland ; Kirby Lonsdale ; Orton. 
Transversely ovate, beak prominent, ribs sharp, extending to the beak ; sides very 
deeply rcflexo-dentate. a. mesial elevation considerable sides, much reflexed, ribs very 
acute, p. (raricosta), plaits few ; 16. a gigantic specimen ; 29, 30 young. 7. (polyo- 
