Small (maximum length 7 mm); profile medium-high; apex anterior to center; anterior slope straight to convex, lateral slopes convex; usually encrusted with coralline algae. Sculpture of fine, sharp radial ribs originating below apex, secondary ribs arising in the interspaces, not reaching thickness of primary ribs. Aperture oval, not crenulate. Color pattern independent of ribbing: most frequently white near apex, gray at margin, with 6 to 10 white rays in a stellate pattern, some rays not reaching margin; some specimens with fine brown lines bordering white rays and fine brown lines that produce a concentric network. Interior translucent white, showing the exterior pattern.
Radula: First pair of lateral teeth closely set at anterior edge of ribbon segment, medial edges convex, lateral edges concave, cusps rounded. Second pair of lateral teeth posterior to first pair, medial edges convex, lateral edges straight, cusps rounded. Third pair of lateral teeth positioned posterior and lateral to second pair, medial edges convex, lateral edges straight to slightly concave. Third laterals broader than second laterals, with lateral extensions to edges of ventral plates; cusps rounded. Marginal teeth lacking. First lateral plates irregular, anterior portions overlapping anterior ribbon segments; second lateral plates elongate, ovoid; third lateral plates triangular, with convex posterior edge. Ventral plates with strong anterior and posterior processes. Lateral portions with strong sutures parallel to edges.
Animal: Pigmentation lacking, oral fringe simple.
HOLOTYPE DIMENSIONS.—Length 4.7, width 3.3, height 2.0 mm.

Notoacmea pumila could be confused only with two other relatively small forms, N. ubiquita new species and Patelloida semirubida. It differs from the first in having much sharper ribbing and not being compressed. Although both N. pumila and P. semirubida have fine sharp radial ribs, N. pumila lacks the sharp concentric sculpture and pink markings of P. semirubida.
The radula of N. pumila is similar to that of two new species described herein, N. ubiquita and N. immaculata. It differs from both by having a complete rather than partial suture between the second and third lateral plates and having strong ventral plate sutures parallel to the lateral edges. The ventral plates of N. pumila have anterior and posterior processes which N. immaculata lacks, and the third lateral plates are triangular rather than biformed as in N. immaculata and N. ubiquita. The radula of TV. pumila differs from that of P. semirubida by lacking marginal teeth.
Large lots show similar color patterns both in the material from Costa Rica and from stations in Ecuador. A small percentage of specimens change with growth from dark rayed to solid dark; fewer specimens are rayed only with brown linear markings and fine brown reticulate markings. Shell proportions vary only slightly.
Notoacmea pumila undoubtedly feeds on coralline algae—the lateral teeth are blunt and of equal size. Living specimens have been collected in the low intertidal zone and the species probably also occurs in the immediate subtidal zone on coralline-encrusted rocks.

TYPE-LOCALITY.—Ecuador: Santa Elena Peninsula; Punta Ancon, north and south sides (2°20'S, 80°54'W), intertidal zone. Leg. J. H. McLean and D. Shasky, 6-7 Mar. 1970 (LACM 70-11, 70-12), 72 LACM specimens, 12 Shasky specimens.
DISTRIBUTION.—El Velero, Nicaragua (12°0rN) (LACM 74-86), south to Ecuador (type-locality). There are numerous dead specimens from Bahia Salinas, Costa Rica (11°02'N, 85°45'W) (LACM 72-17, 72-19); two specimens only from Panama at San Carlos (8°29'N, 79°57'W) (LACM 75-55), and a number of localities in Ecuador collected by D. Shasky.