I am by no means persuaded by the distinctiveness of the two subspecies of this taxon since it has seemed to me that there is a great deal of overlap in leaf characteristics which are the main discriminants, but the Jepson Manual accepts both subspecies, so I will list them separately with pictures that illustrate those differences.  Miner's lettuce is a member of the Purslane family along with Calandrinia, Calyptridium, Lewisia, Montia and Portulaca, and is a short-stemmed, erect, somewhat succulent annual that grows to about 12" tall.  Commonly found in shady places in coastal sage scrub, chaparral and southern oak woodland throughout the California Floristic Province, its flowers are in an inflorescence that is either sessile or stalked above an orbicular leaf. There are five whitish petals, five rounded sepals, and a single style with three stigmas. It is the shape of the leaf that is supposed to distinguish between ssp. perfoliata and ssp. mexicana, with this subspecies having basal leaves that are round-deltate to reniform, usually wider than long, with a truncate base and a short-pointed tip, and cauline leaves that have two short abrupt points, while perfoliata has basal leaves that are elliptic to round-deltate, usually longer than wide, with a wedge-shaped base and an obtuse to acute tip, and cauline leaves that are round or have small obtuse tips. Miner's lettuce may be found blooming from February to June

Click here for Latin name derivations: 1) Claytonia 2) perfoliata 3) mexicana.
Pronunciation: klay-TONE-ee-a per-fo-lee-AY-ta mex-i-KAY-na.
Click here for Botanical Term Meanings.