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PALEONTOLOGY: R. RUEDEMANN 
THE PHYLOGENY OF THE ACORN BARNACLES 
By Rudolf Ruedemann 
State Museum, Albany, N. Y. 
Communicated by J. M. Clarke, October 29, 1918 
The Acorn Barnacles of Balanidae represent the most advanced stage of 
regression attained in the Cirripedes through adaptation to a sessile life. 
Darwin regarded them as derived from the Lepadidae or Goose Barnacles by 
the loss of the peduncle, and this view has been generally accepted. The 
whole order of Cirripedia is interpreted by some students as derived from the 
Copepoda, but others find its ancestors in the ostracodes, because of the bi- 
valved 'Cypris-stage' through which the larva passes after the Nauplius-stage 
and directly before attachment. Balfour, however, in his treatise on Com- 
parative Embryology, 1880, p. 424, emphasizes the presence of paired, com- 
pound eyes as well as the large bivalve shell, and has urged their "independ- 
ent derivation from some early bivalve Phyllopod form." 
It has further been recognized that a reduction in the number of the com- 
partments (wall plates) or lateralia has taken place, probably by coalescence. 
The typical Balanus has four lateralia, two on each side; besides these the 
rostrum and carina, or six compartments in all. But the most generalized 
living genus Catophragmus has eight compartments, or three lateralia on 
each side. Paleontologists know in the Devonian fauna Protobalanus hamil- 
tonensis Whitfield, an acorn barnacle with 12 compartments or 5 lateralia 
on each side 
Investigations of the Utica and Lorraine faunas of the Upper Ordovician 
have recently brought to light a new form of acorn barnacle, which like Pro- 
tobalanus, has five lateralia on each side, but differs in the form of these later- 
alia in such a way as to shed a most important light on the problem of the 
origin of the barnacles. In Protobalanus the five lateralia, which are all 
acutely triangular plates, point uniformly upward or inward towards the 
mouth. In the new form, which will be described as a species of the new 
genus Eobalanus, these plates are arranged as shown in the diagram, the four 
outer lateralia as in Protobalanus, while the middle one is inverted, forming a 
sort of keystone in the series. The second and fourth are furthermore some- 
what truncated at the apex. 
The effect of this peculiar shape of the lateralia is that, if they could be 
matched together like the parts of a picture puzzle, they would give a perfect, 
snugly-fitting carapace, as shown in diagram V. This carapace is bivalved 
as in phyllopods, with the rostrum and carina of the barnacle corresponding 
to the rostral plate of the crustacean in front and the 'dorsal plate' behind, 
exactly as in the Devonian genera Mesothyris and Rhinocaris. 
We have then to picture the derivation of an Eobalanus as from a Rhino- 
caris-like phyllopod as illustrated in the set of diagrams. The first diagram 
