Lottia insessa: Shell. High profile; apex positioned in the anterior third of shell. Anterior slope straight; posterior and lateral slopes convex. Lateral edges of the shell parallel. Sculpture of fine riblets and concentric growth lines. Apex dark brown, with or without white markings; remainder of shell glossy brown. Interior of shell brown. Length, 5-20 mm.
Animal. Dark dorsal mantle margin.
Radula. First lateral teeth closely set at anterior edge of ribbon segment, pointed distally, medial edge convex, lateral edges concave. Second lateral teeth broad, pointed distally; third lateral teeth not reduced, pointed distally, lateral edge concave forming a lateral extension of tooth. Lacks marginal teeth. First lateral plates rectangular; second lateral plates with straight posterior edge; third lateral plates semicircular. Second and third lateral plates fused. Ventral plates subrectangular with broad, flat anterior process.

Lottia insessa: Shell rather thin but strong; outline oval or oblong, the sides often parallel; elevated, conical, the apex in front of the middle, slopes convex ; surface smooth, polished. Color varying from yellowish or olive-brown to chocolate ; inside usually very deep brown with a lighter border. Apex blackish, sometimes having snowy dots or two crescents, the horns of one directed forward, of the other backward. Length 20, breadth 11, alt. 12 mm.

Lottia insessa: A small, smooth, dark species. Faint, almost obsolete, radiating lines are usually perceptible. It is much larger than A. paleacea or depicta and is not so narrow. Compared with A. asmi it is larger, longer, and brown instead of black.
Lottia insessa is found on the stipes of Egregia, where it erodes a furrow as it feeds on the cortical cells of the plant. Some of these furrows may be more than 1 m in length. The eurytopic species Lottia pelta also erodes a furrow on stipes of Egregia. Although Egregia is found at numerous sites in San Francisco Bay, L. insessa has been found only at 1 locality near the mouth of the Bay. Lottia pelta, on the other hand, is found at numerous localities and may be confused with L. insessa when it occurs on Egregia. Lottia insessa is distinguished from L. pelta by the brown, rather than white, intermediate area. Lottia insessa also has a dark, dorsal mantle margin; in L. pelta the dorsal mantle margin is unpigmented. The lateral extension of the third lateral teeth of L. insessa are not found in any other California acmaeid.

Lottia insessa: Northeast Pacific. Sitka south to San Diego, California. The Bay of Monterey is probably the central point for this species, in regard to numbers of individuals. It lives on the fronds of seaweeds.

Wrangell, Alaska (56°20'N) to Bahia Magdalena, Baja California, Mexico (24°30'N) (McLean, 1966:108). San Francisco Bay: San Francisco County—St. Francis Yacht Club Breakwater.