218 
DESCRIPTION OF THE FOSSILS. 
The triangular upper valve, narrow mesial fold, and few lateral radii, agree better 
with Sp. triangularis than trigonalis of Sowerby. 
S. trigonalis. Sow. t. 265. Kirby Lonsdale ; Arran ; Derbyshire ; 
Northumberland. 
Differs from the last by having (when full grown) more lateral ribs, (about 
twenty-four in all), sharply striated across ; and a less distance between the beaks. 
S. octoplicata. Sow. t. 562. Kirby Lonsdale ; Pateley bridge ; Der- 
byshire. 
My specimens are not good. Its few strong plaits distinguish it from Sp. 
triangularis. 
S. attenuata. Sow. PI. IX. fig. 13. Bolland ; Queen’s County. 
Width about double the length ; cardinal area narrow, sulcate ; cardinal extremi- 
ties elongate ; surface radiated with numerous smooth obtuse ribs. — Beaks approxi- 
mate. ( 13 . In a beautiful specimen from Kildare County, given me by Dr. Hutton 
of Dublin, the sides run out into long spines. It is probably a distinct species.) 
S. bisulcata. Sow. PI. IX. fig. 14. Bolland; Queen’s County; Coal- 
brookdale, Northumberland. 
Differs from the last almost only in the proportion of length to breadth, and in 
having fewer radiations. The furrows which bound the mesial fold on the upper 
valve are scarcely more prominent in this than in several others. Some of the 
Coalbrookdale specimens are quite as long as wide. 
S. rotundata. Sow. PI. IX. fig. IT. Bolland ; Queen’s County ; Kildare. 
Cardinal area narrower than in any of the preceding : beaks approximate, radiated 
with very obtuse or obscure sulci ; mesial fold broad, nearly smooth. The Irish and 
Yorkshire specimens agree perfectly: the cardinal extremities are seldom so angular 
as in figure 17, from Mr. Gilbertson’s collection. 
S. pinguis. Sow. PI. IX. fig. 18, 19- Bolland; Castleton. 
In the greater distinctness of the lateral radii it differs from the preceding, but it 
may, very possibly, be only the young of that. 
S. humerosa. Ph. PI. XI. fig. 8. Greenhow hill. 
Lower valve extremely large, swollen near the beaks, and produced in a mesial 
furrow, receiving the angular ridge of the upper valve : radiating ribs small, duplicate. 
C. Radiates ; cardinal area not so wide as the shell ; surface radiated. 
S. duplicicosta. Ph. PI. X. fig. 1. Bolland ; Derbyshire ; Northum- 
land. 
Mesial fold angular ; radiating ribs numerous, duplicate toward the margin. 
