Shell oval lenticular, with an erect apex, white, somewhat solid and slightly translucent. The protoconch of the holotype was examined under magnification, and it has a nucleus with a diameter of 0.082 mm and about 1,5 whorls, but it is difficult to see the microsculpture, because it is a little eroded. For this reason, in the description it was mentioned as smooth. The mention of two whorls of the protoconch depends on the method of measurement employed. The protoconch in the Cuban material begins with a nucleus of about 0.070 mm. This nucleus and the first half whorl (embryonic) are smooth, followed by 4 low, very fine spiral threads. The end of the protoconch is not clearly defined, because the axial ribs are preceded by a 1/2 whorl that could belong either to the protoconch or to the teleoconch. The colour is white. The beginning of the teleoconch is not well defined. There is a portion of the preceding 1/2 whorl where the threads change into grooves. This part is considered by us as teleoconch; after this 1/2 whorl, the axial ribs begin. The teleoconch has 1,5 whorls, with rapid development; it is sculptured by strong axial ribs which are oblique and S-shaped. They are very variable, numbering, on the less sculptured shells, about 13 on the first whorl and between 15 and 18 on the body whorl. Many spiral striae are present on the entire shell and can be seen to continue over the ribs. Aperture ovoid, with the outer lip a little angulated towards the periphery. The early whorls are visible in the large umbilicus. Many specimens have the aperture totally separated from the previous whorl. Dimensions: From 1.5 to 2.2 mm in diameter.
The radula was obtained from a specimen from Cienfuegos, Cuba: the rachidian tooth presents three small denticles on each of the basal supports; the lateral teeth are trapezoid in form, wide basally, with the sharp area also wide, where there is a central gross and blunt denticle, 8-9 smaller denticles on its external border and 4-5 on the internal one. Marginal internal teeth oar-shaped, strongly curved and pointed, with their external margin smooth and slight incisions in their internal margin.

The morphology of the shell and radula confirm that it belongs to the genus Macromphalina. At the beginning of this study we had some doubts about the conspecificity of the holotype of M. palmalitoris and the material collected in Cuba (similar to the form figured from Puerto Rico by WARMKE AND ABBOTT, 1961), because the number of axial ribs was lower in the Cuban shells but higher on the holotype: this can be shown by comparing the last 1/4 whorl of the holotype (with 18 ribs), to most of the shells from Cuba, with only 8. The sculpture of the protoconch was not compared because of the bad condition of that of the holotype. Nevertheless, we did not find differences in most of the other characters: the number of whorls of the protoconch, the beginning of the teleoconch, and the sculpture and profile of the shell. So, finally, we decided to consider the specimens from Cuba merely a form, especially because we found some specimens which seemed to represent intergrades.

The species was described from Florida. We have studied material from Cuba, and WARMKE AND ABBOTT (1961) drew a shell from Puerto Rico. So the species is probably distributed at least in the middle Caribbean.