_Ischnochiton conspicuus_, Cpr. is common at San Pedro and is usually found on the under side of rocks in sandy tidepools. In this situation they grow quite regularly, showing but slight variation in form. They are active fellows and evidently sensitive to light; for if the rocks to which they are clinging are turned over, they soon glide away, always taking the shortest route to the under side of the rock. While cleaning a lot of this species taken at San Pedro last fall two specimens were noticed which differed so widely from the others that they might easily have been mistaken for another species. They were much wider and lower-arched than the typical form and the posterior corners of the valves were rounded off, making the lateral areas very narrow. Hoping to find more specimens of this odd form, a trip to Point Firmen was devoted wholly to the collection of chitons, with interesting results. The usual species were found in the tidepools including numerous specimens of _Ischnochiton conspicuus_ of the ordinary form. Ledges of soft rock beyond the tidepools contained many old pholad holes and in these were found the form for which I was searching. Twenty specimens of various sizes were collected from as many pholad holes. In the larger specimens the foot had become so greatly enlarged to fit the concave bottoms of the holes that it was impossible for them to curl up in the usual manner. Some of these specimens were so badly eroded by the sand and gravel which wash in and out of the holes that the anterior valve was reduced to two thirds of its normal height. In color pattern, sculpture, and mantle characters these specimens were identical with those from the tidepools, and, as will be seen by referring to the table of measurements, the smaller specimens approach quite closely to the proportions of the typical or tidepool forms. TYPICAL OR TIDEPOOL FORMS. ║ SPECIMENS FROM PHOLAD HOLES. (Lot 1) ║ (Lot 2) _Lon._ _Lat._ _Index._ _Div._ ║ _Lon._ _Lat._ _Index._ _Div._ 99 35 2.52 130°║ 65 33 1.97 135° 83 33 2.51 130°║ 63 32 1.97 155° 83 30 2.76 125°║ 62 27 2.29 140° 62 22 2.80 125°║ 60 30 2.00 130° 59 23 2.56 130°║ 56 29 1.93 130° 38 14 2.71 130°║ 42 17 2.47 130° In brief, Lot 1 shows an angle of divergence constant at 125° to 130°, where Lot 2 shows an angle varying from 130° to 155°, and a proportion of length to width 2.51 to 2.80 as against a proportion ranging from 2.47 to 1.93.