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When young they bear small fleshy leaves along with the spine-tufts; but the former fall off at an early stage, whilst the spines are altered in length or number as the joints get old. In one or two kinds the spines fall away when the joints begin to harden, and in O. subulata the leaves are large and persistent.
The terrestrial shells are all univalves. Many fossil species formerly referred to this genus, and which have been met with chiefly in marine formations, are now considered by conchologists to belong to Natica and other marine genera. Pleurotoma exorta, Brand. Upper and Middle Eocene. Ancillaria subulata, Sowerby. Barton clay.
This handsome South American species was the subject of an interesting communication to the Gardeners' Chronicle, in 1884, from Dr. Engelmann. It had previously been known as a Pereskia from the fact of its leaves being persistent and very large. In its leaves, flowers, and seeds, O. subulata is one of the most interesting of the genus.
Besides these lithophytes, some fragments of the Millepora alcicornis, which occurs in the same relative position at Keeling Island, were brought up; and in the deeper parts there were large beds of a Seriatopora, different from S. subulata, but closely allied to it.
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