Sheaths of the leaf-clusters deciduous; cones ½′—2′ long, maturing in the third year, leaves in 3-leaved clusters, slender, 2½′—4′ long. 9. P. leiophylla (H). Sheaths of the leaf-clusters persistent. Leaves in 3-leaved clusters (3 and 5-leaved in 10, 3—2 leaved in 12). Cones subterminal, usually deciduous above the basal scales persistent on the branch. Buds brown; leaves in 2—5-leaved clusters. 10. P. ponderosa (B, F, G, H). Buds white. 11. P. palustris (C). Cones lateral. Cones symmetrical, their outer scales not excessively developed. Leaves in 2 and 3-leaved clusters, 8′—12′ long; cones short-stalked. 12. P. caribæa (C). Leaves in 3-leaved clusters; cones sessile. Cones oblong-conic, prickles stout; leaves 6′—9′ long. 13. P. taeda (A, C). Cones ovoid, prickles slender; leaves 3′—5′ long. 14. P. rigida (A, C). Cones unsymmetrical by the excessive development of the scales on the outer side. Cones 5′—6′ long, their scales not prolonged into stout, straight or curved spines. Prickles of the cone-scales minute. 15. P. radiata (G). Prickles of the cone-scales stout. 16. P. attenuata (G). Cones 6′—14′ long, their scales prolonged into stout, straight or curved spines; leaves long and stout. Cones oblong-ovoid; seeds longer than their wings. 17. P. Sabiniana (G). Cones oblong-conic; seeds shorter than their wings. 18. P. Coulteri (G). Leaves in 2-leaved clusters (2 and 3-leaved in 23). Cones subterminal. Cones symmetrical, 2′—2½′ long, their scales unarmed; leaves 5′—6′ long. 19. P. resinosa (A). Cones unsymmetrical by the greater development of the scales on the outer side, armed with slender prickles; leaves 1′—4′ long. 20. P. contorta (B, F, G). Cones lateral. Cones about 2′ long. Cone-scales very unevenly developed and mostly unarmed; cones incurved; leaves less than 2′ long. 21. P. Banksiana (A). Cone-scales evenly developed, armed with weak or deciduous prickles; leaves up to 4′ in length.