Leaf-bases long-sheathing, expanded by the separation of the fibers of the side opposite the midrib; petiole strongly flattened above the base, prominently angled above and below; ligule large and firm, produced laterally to support the outer divisions.
Leaf-divisions narrow, separated below the middle and below the point of greatest width; texture firm and coriaceous; veinules subequal, close together, cross-veinules obsolete. Lower surface clothed with persistent closely appressed hairs, the upper coated with wax when young.
Seeds with few longitudinal grooves, the surface not polished, grayish; embryo subapical.
The generic name alludes to the preference of this palm for the summits of crags and the brows of perpendicular cliffs which abound in the limestone region of the north side of Puerto Rico.
The tall, slender trunk and other differences between this genus and Thrinax are probably to be interpreted as ecological adaptations necessary to enable the present palm to compete with the vegetation which often surrounds its base, and to withstand the winds to which it is commonly exposed. The species of Thrinax and other allied genera, as far as known, have the trunk rigid and columnar, or even enlarged from the base upwards. When growing solitary and exposed they seldom, if ever, attain half the height of Thrincoma. Usually, however, they are protected by other vegetation or by growing gregariously in thickets.
Thrincoma might be described as a Thrinax which has adopted habits of the arecoid genus Acria which grows in similar situations in a neighboring part of the island. In addition to the smooth, slender, and flexible trunk Thrincoma makes further provision against the wind in having fewer, less ample, tougher and more deeply divided leaves and like the arecoid palms it also drops the old leaves as soon as their usefulness is past, instead of retaining, like Thrinax, a large pendant cluster of them. The details of these differences are given below in a comparative note on fresh material of Thrincoma alta and Thrinax praeceps collected but a short distance apart in the lower part of the Arecibo valley along the Utuado-Arecibo road. In this region of jagged mountains, Thrinax seeks shelter against the walls of perpendicular precipices, while Thrincoma challenges the wind and the admiration of the traveller by its evident preference for the crags and pinnacles.
Thrincoma alta sp. nov.
With but one species known with certainty to belong to the present genus the separation of generic and specific characters would have little purpose. Data for a specific description are, however, contained in the following notes which are retained in their original comparative form as better illustrating the generic differentiation of Thrincoma and Thrinax, as represented by Thrinax praeceps.
The trunk of Thrincoma differs in three adaptive particulars from that of Thrinax praeceps, Ponceana and similar species which are merely columnar with very short internodes and an irregularly rimose surface, not smooth and hardened.
1. There are distinct internodes from 3.5 to 5 cm. in length. These indicate rapid growth and would increase the chances of survival in the face of competition of quick-growing tropical vegetation.