218 Bocene Fossils of the United States. 
flattened wmbo and acute apex which is scarcely oblique. It 
does not agree with either Scissodesma or Mulinia. 
In a fossil state we findvonly the two following species. I have 
seen none recent. . 
TriqueTra £quorEa. ‘Triangular, equilateral, thick, plano- 
convex; umbonial slope submarginal, angulated ; umbo fiatten- 
ed, apex acute; anterior extremity acuitely rounded ; posterior 
extremity obliquely truncated ; cardinal plate very thick ; fosset 
small, ovate; cardinal plate thickest under the anterior cardinal 
tooth. (Plate I, fig. 5.) 
Erycina e@quorea, Con.; Foss. Shells of Tert. Form. p. 42, 
Oct. 1833. 
Mactra Grayi, Lea; Cont. to Geol., Plate I, fig. 10. 
Claiborne, Alabama. 
A common species, the valves of which are always found sep- 
arated. The substance of the shell is remarkably thick. 
TRIQUETRA RECTILINEARIS. ‘Triangular, flattened above, with 
coarse lines of growth; posterior side subcuneiform, rather longer — 
than the anterior side, extremity subtruncated ; umbo much flat- 
tened, apex. acute; posterior dorsal margin — basal 
margin a little tumid near the middle. (Plate II, fig. 8.) 
Errycina rectilinearis, Con. ; Foss. Shells of Ter, Form. p. 42. 
Claiborne, Alabama. 
This shell is larger, thinner, more inequilateral than the pre- 
ceding, and very rare. The cardinal plate is less thickened and 
the anterior muscular impression not so deeply impressed. » Both 
species have an indistinct impressed line on the exterior, extend- 
ing from the anterior end to the apex. 
Explanation of Plates. 
Prate I. Prate Il. 
Fig. 1. Siliquaria vitis. _ Fig. 1. Pholas petrosa. 
2. Dentalium thalloides. 2. Panopza elongata. 
3. D. arciformis. 3. Mactra decisa. 
4, Anguinella ornata. 4. M. preetenuis, 
5. Fistulana larva. 5. Triquetra equorea. 
6. Balanus peregrinus. 6. Mactra parilis. 
7. Lutraria lapidosa, 7. Crassatella protexta. 
8. L. papyria. 8. Triquetra rectilinearis. 
9. Pholadomya marylandica. 
