This, the typical genus of the family, includes 39 recent species, which range from 77° in the northern hemisphere to Cape Horn in the south.
Sub-genus, dcasta, Leach, 1817.—Compartments six ; parietes and basis non-porose ; basis calcareous, cup-formed, not elongated : attached to sponges or rarely to the bark of Isis.
This sub-genus, which is a very natural one in habits and appear ance, nevertheless can hardly be distinguished from those species of Balanus which live attached to Gorgonio. Acasta is found in almost all parts of the world.
Tetraclita, Schumacher, 1317 (Copia of Leach, Asemus of Ranzani). —Compartments four, sometimes with their outer laminae calcified together ; parictes permeated by pores, generally forming several rows ; basis flat, irregular, calcareous or membranous.
membranous ; scuta narrow, united to the terga by a horny articular ridge.
Two of the three species included in this genus are always attached to turtles ; the third adheres to crabs and smooth shells.
Section it Scutum and tergum (when both are present) not overlapping each other ; basis membranous ; parietes often deeply folded, with the outer lamina towards the basis generally imperfect; each branchia composed of two plicated folds : shell attached.to living Vertebrata.
Coronula, Lamarck, 1802 (Diadema of Schumacher ; Cetopirus of Ranzani).—Compartments six, of equal breadth, deeply folded, with the folds outwardly pressed together, but inwardly expanded, so as to form cavities open only On the under side ; opercular valves much smaller than the orifice of the shell : attached to Cetacea.
Ebniniva, Leach, 1825.—Compartments four ; parietes not porose; basis membranous.
This genus is confined to the southern hemisphere.
Pyrgoma, Leach, 1817 (Jfegatrema of Leach; Adna of Leach ; Daracia of J. E. Gray ; Creuaia of De Blainville ; Nobia of Sowerby). —Shell formed of a single piece; basis cup-formed or sub-cylindrical : attached to, or imbedded in corals.
Platylepaz, J. E. Gray, 1825 (Coronula of De Blainville).—Compart ments six, each bilobed and inwardly produced, so as to form six mid-ribs, which support the outwardly convex membranous basis.
The species of this genus are attached to turtles, manatees, and sea-snakes.
Tubicinella, Lamarck, 1802 (Coronula of De Blainville).—Compart ments six, of equal breadth ; shell sub-cylindrical, wider at the top than at the basis ; belted by several transverse ridges : attached to Cetacea.
Sub-genus Creusia, Leach, 1817.—Compartments four ; furnished with radii; basis cup-formed, imbedded in corals.
This sub-genus is most closely allied to Pyrgoma, and its separation is of doubtful propriety.
Chtionobia, Leach, 1817 (Coronula of Lamarck ; Astrolepas of Gray). —Compartments extremely thick, six in number, but the rostrum is internally composed of three compartments united together ; basis %enact/anus, Steenstrup, 1852.—Shell almost rudimentary, star formed, composed of six compartments, with a long peduncle-formed body rising from the middle of them ; opercular valves none attached to Cetacea.
Sub-Family.—Obtleamalime.
Shell with the rostrum having siva, but without radii ; rostro-lateral compartments without aim on either side ; parietes not porose.
Chtharnalus, Ranzani, 1820 (Euraphia, Conrad).—Compartments six; basis membranous, but sometimes in appearance calcareous from the inflected parietes.
aiariarsipho, Darwin, 1854.—Compartments four, with the sutures often much obliterated ; basis membranous.
Pachylanna, Darwin, 1854.—C,ompartments, when the shell is very young, eight ; when maturer, either six, or in appearance only four, from the close union of the lateral compartments ; basis calcareous.
Almeria, G. B. Sowerby, 1825.—Compartments eight ; radii with their edges crenated ; basin membranous.
Verruca, Schumacher, 1817 (Clisia and Clitia of Leach, decade of Lamarck, Ochthosia of Itanzani).—This games is very remarkable in many respects, especially in its asymmetrical shell ; sometimes the right side and sometimes the left side being specially modified. It includes four recent species.
a, the outside ; below, an internal view of the eight divisions; c, the anterior piece; d, the posterior piece ; • to k, the lateral pieces ; 1, the opereulum, consisting of four pieces, of which the two anterior are the larger.