Katy. Below she wrote:

You will find Tunas in the cacti thickets of any desert, but if
you are so fortunate as to be able to reach specimens which were
brought from Mexico and set as hedges around the gardens of the
old missions, you will find there the material for this salad in
its most luscious form. Naturally it can be made from either
Opuntia Fiscus-Indica or Opuntia Tuna, but a combination of these
two gives the salad an exquisite appearance and a tiny touch more
delicious flavor, because Tuna, which is red, has to my taste a
trifle richer and fuller flavor than Indica, which is yellow.
Both fruits taste more like the best well-ripened watermelon than
any other I recall.

Bring down the Tunas with a fishing rod or a long pole with a
nail in the end. With anything save your fingers roll them in
the sand or in tufts of grass to remove the spines. Slice off
either end, score the skin down one side, press lightly, and a
lush globule of pale gold or rosy red fruit larger than a hen's
egg lies before you. With a sharp knife, beginning with a layer
of red and ending with one of yellow, slice the fruits thinly,
stopping to shake out the seeds as you work. In case you live in
San Diego County or farther south, where it is possible to secure
the scarlet berries of the Strawberry Cactus-- it is the
Mammillaria Goodridgei species that you should use--a beautiful
decoration for finishing your salad can be made from the red
strawberries of these. If you live too far north to find these,
you may send your salad to the table beautifully decorated by
cutting fancy figures from the red Tuna, or by slicing it
lengthwise into oblong pieces and weaving them into a decoration