Auckland Harbour.—A long reef of volcanic rocks stretches from west of Ponsonby towards Kauri Point, fairly across the tidal currents. Rivers flowing into the upper reaches of the harbour bring down much mud and decayed vegetable matter, a great deal of this being caught by the reef, which, except where exposed to the scouring action of waves and tides, bears a coating of slimy mud. It is not to be wondered at, therefore, that animal associations rich in individuals and species are found near low-tide
mark near the outer extremity of this reef. Two such communities are very conspicuous: one is that of Vermilia carinifera, quite the most remarkable of its kind I have seen; the other is an association in which the tunicate Cynthia is the dominant form. This species occupies a belt of rocks extending from low-water mark at spring tides to nearly 1 m. in vertical height above. It occurs usually in clusters, some twenty or thirty being united, and attached to the rock or to an adjoining cluster at five or six places only. The depth of the covering thus formed is about 5 cm. (Plate 42, fig. 3.) The tunicate does not completely, cover the rock, but occurs in patches 10 cm. to 20 cm. in diameter. The angles of the rocks are preferred, and small irregular rocks are quite covered, but the smooth rocks are usually more or less bare. The fact that the individuals in a cluster can be separated, though with difficulty, shows that the mode of forming clusters is simply by the young settling on the outside of the tests of older individuals and growing there. Many young can be found in such positions. The tests, where touching, are joined together very firmly by their stringy and glutinous exteriors, and thus compact clusters are formed. Another larger species of tunicate occurs singly along the lower portion of the association.
Frequently embedded in the masses of Cynthia occur little colonies of Musculus impactus, each spinning round its shell a nest resembling a cocoon. Growing on the tunicate clusters are many plants of a small brown algae which swarms with Rissoina olivacea. Many gasteropods are found on and among the clusters; most common are Euthria vittata, Turbo smaragdus, Murex octogonus, and Cerithidea subcarinata, which swarms over rocks, algae, and tunicates alike. The half-crab Petrolisthes elongatus is extremely common, hiding among the tunicate clusters.
On rocks among the tunicates are many animals and plants. The globose sponge Tethya, and an encrusting bright-orange species, are common. Corallina officinalis is abundant, and collects a lot of mud. It appears to be the most favoured resort of Cerithidea subcarinata. The larger gasteropods include Turbo smaragdus, with fairly clean shells and eroded spire; Cominella adspersa, a very distinct-looking form with a pronounced shoulder beneath the suture, yellow inside, and usually extensively eroded without; Verconella adusta, fairly clean, though sometimes carrying a few small Elminius. Ostrea corrugata up to 7 cm. in length occurs singly or in small clusters. Patiriella regularis is common, and apparently always of a reddish-brown colour. Mytilus canaliculatus occurs singly in crevices. The shells are usually almost covered with Calpidula crepidula, and occasionally also some Calyptraea novae-zealandiae, Amaurochiton glaucus, and Ischnochiton maorianus.
The under-side of stones is equally as rich as the upper surface. Near the mud-line are Ischnochiton maorianus and Lepidopleurus, iredalei; but where the water circulates freely, though light does not fall direct, there are Cynthia, but no algae, and numerous gasteropods, sponges, compound ascidians, sea-anemones, crabs, pelecypods, Flabelligera lingulata, Nereis arnblyodonta, and other worms of various kinds. (Plate 47, fig. 1.)