LORQUINIA f3 
Ischnochiton cooperi, Cpr. Placiphorella velata, Cpr. 
— 2 fragments. — 1 fragment. 
magdalenensis, Hds. Tonicella lineata. Wood. 
— 9 valves, 12 fragments. — 1 valve. 
The outstanding features of this hst of shells are (1st) its some- 
what contradictory stratigraphic evidence, (2nd) its richness in chitons, 
and (3rd) its limitations, or should we say its purity? 
In Dr. Arnold's book to which we have already referred, he 
divides the Pleistocene deposits of San Pedro and vicinity into two 
groups, which he calls the Upper and Lower San Pedro series. This 
latter is a gray sandstone, usually quite hard, and we have found no 
reference to any such deposit as Upper San Pedro series. On the 
other hand, of the twelve species of the above list which Arnold found 
in only one stratum, one belongs to the Pliocene, even older than the 
Lower San Pedro, four to the Lower San Pedro itself, and seven to 
the Upper San Pedro. Of the nineteen species which we have taken 
and which are not recorded by Arnold, practically all are still living 
here, although more common in the north. It is sincerely to be hoped 
that some of our Southern California geologists will take an early 
opportunity to seek decisive evidence of the age of this particular 
deposit. 
The next point of interest, in fact the thing which first impresses 
the collector, is the large number of chiton valves which this deposit 
yields. Dr. Arnold obtained three valves in the course of his years of 
work. Mr. Oldroyd reports a single valve (Ischnochiton conspicuus) 
from that rich pocket of Pleistocene shells at Los Cerritos. Dr. Clark 
has obtained something over 200 specimens from his long and vigorous 
work above Santa Monica, but having had the pleasure of sorting 
over some of his material, we have no hesitation in saying that they 
do not represent nearly so large a proportion of the shells handled 
as do the ninety valves and identifiable fragments which we have taken 
from this exposure. 
Closely associated with the large number of chiton valves is the 
unusual homogeneity of this list. It contains just three strictly sand- 
dwelling species : Tornatina eximia and the two Alectrionidae, while 
a list of equal length from Santa Barbara included twelve. This purity 
combines with the small extent to suggest a possible origin for the 
deposit. We can easily imagine a small tide-pool near the low-water 
line of a rocky coast. In and close around it lived these fifty-odd 
species, as most of them are found today, until these strangers were 
ushered in by a heavy storm. Is it too great a strain to picture this as 
occurring at the beginning of the period of shallow water conditions 
which filled the little pool with the gray sand, and then sealed it with 
the red which still lies above it? 
