oe) NEW YORK STATE MUSEUM 
but 6 plates. As a matter of fact paleontology furnishes in Proto- 
balanus a form with 12 compartments arranged exactly as in Eoba- 
lanus, viz: in 5 pairs of lateralia and a small rostrum and larger 
carina. While however in Protobalanus the acutely triangular 
lateralia are all pointing with their apexes inward or upward, there 
prevails in Eobalanus the peculiar interlocking arrangement de- 
scribed above. This, in our view not only constitutes the generic 
difference between the two genera, but also gives us an ONE 
clue as to the ancestry of the Balanidae. 
The effect of the peculiar shape of the lateralia in Eobalanus 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 of text-figure 1. Leaving the two terminal plates sepa- 
rate, we thus obtain the outline of a bivalved carapace, 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 phyllocarids Mesothyra and Rhinocaris. (See 
diagram of Rhinocaris, text-figure 1.) 
We have then to picture the derivation of an Eobalanus from a 
Rhinocaris-like ancestor as illustrated in the set of diagrams. 
The first diagram 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 ; and presum- 
ably also for many of their probable descendants, the phyllocarids. 
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 between the valves (stage II of 
diagrams), thus becoming the rostrum and carina of the barnacles. 
Following this was the breaking up of the valves into the lateralia, 
owing to stresses exerted at one end or the other, possibly the 
anterior one, where the originally chitinous and somewhat flexible 
valve was attached. It is 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 cara- 
pace have been divided up without leaving a useless remainder. 
