Theodore W. Richards

Abstract: shows the carapace of the crustacean just before attachment with the head portion and the dorsal side downward-a rather natural position for phyllopods, which, like Apus, are wont to swim on their backs, while foraging along the bottom. The head and back being thus protected by attachment, but the ventral side open to attack, the next step will be the separation of the carapace valves along the hinge line and their movement upward towards the ventral side; and likewise the rostral and dorsal plates will have to move upward to fit in again between the valves (stage II of diagrams). Following this was the breaking up of the valves into the lateralia, owing to stresses exerted at one or other end, possibly the anterior one where the originally chitinous and somewhat flexible valve was attached. Here also, our material affords a clue to the mode of procedure. A very early growth stage of Eobalanus shows four radially arranged, subequal, oval plates, the two lateral ones of which show a suture along which a smaller part is being split off. It is thus to be inferred that the compartments were formed by successive splitting off of plates from the original valve, each fissure producing a new pair of lateralia. In this way the peculiar interlocking arrangement of the compartments in Eobalanus would finally have come about and each valve of the carapace have been divided up without leaving a useless remainder. The scuta and terga which form the valvular carapace or operculum of the upper aperture of the later Balanidae and Lepadidae and which are of great taxonomic importance, have not been found in Eobalanus and Protobalanus, and in our view did not exist then, but are a later development to close in