128. _The Conchoid Skeleton._--By the name "Conchoid skeletons" are distinguished the bivalved lattice-shells which occur exclusively in the legion PHÆODARIA; they are quite characteristic of the #Phæoconchia# or _Phæodaria bivalvia_, which embrace three families:--Concharida (Pls. 123-125), Coelodendrida (Pls. 121, 122), and Coelographida (Pls. 126-128). The two valves of the lattice-shell of the Concharida are simple, hemispherical, or boat-shaped, whilst in the Coelodendrida and Coelographida tubes grow out from them, which branch and usually give rise by anastomosis to a second external bivalved shell. In all #Phæoconchia# the two valves are so disposed about the central capsule that an open slit remains between them, into which open the apertures of the central capsule; and since all these _Phæodaria conchoidea_ are TRIPYLEA, with three typical openings in the central capsule, and since the two lateral accessory openings lie at either side of the aboral pole, and the unpaired main-opening at the oral pole of the main axis, it follows that the two valves are to be regarded as dorsal and ventral as in the Brachiopoda (not right and left as in the Lamellibranchiata). The dorsal and ventral {lxxxv}valves are usually equal, but in a portion of the Concharida they present constant differences. In this family the two valves are attached to each other by their free edges, just as in the bivalved Mollusca and Diatoms; and these edges may either be smooth (Conchasmida, Pl. 123, figs. 1-6), or dentate (Conchopsida, Pls. 124, 125); the valvular connection of the latter is sometimes strengthened by a special ligament which unites the two valves at the aboral pole (Pl. 123, figs. 8, 9). The form of the valve is sometimes hemispherical, sometimes boat-shaped, with a sagittal keel.