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. 2. The trunk tapers gradually from a diameter of 9 cm. near the base to 3.5 at the top, and thus possesses considerable flexibility in view of its great length, 11 meters, _Thrinax praeceps_ and other related types not exceeding 4 or 5 meters.