652 ARTHROPODA SUB-KINGDOM VII 
six pieces. Opercular plates sub-triangular ; base membranous or calcareous. Eocene 
to Recent. 
Protobalanus, Whitf. Composed of twelve plates, of which the carina is the 
largest ; rostrum small, lateralia 
in five pairs, fused only near the 
base. Middle Devonian. 
Acasta, Leach. Shell com- 
posed of six solid pieces. Base 
calcareous, cup-shaped ; attached 
to sponges and Alcyonarians. 
Pliocene and Recent. 
Pyrgoma, Leach (Creusia, 
Blainy.). Shell formed of a single 
piece. Base cup-shaped or sub- 
cylindrical ; attached to corals. 
Lower Devonian. Tertiary and 
Recent. 
Pulaeocreusia, Clarke (Fig. 

Fic. 1366. : in ‘ : é 
Balanus concavus, Bronn, Crag; Sutton, England, A, Shell. 1368). Shell in One Pia ith 
B Tergum. C,Scutum. 1); (after Darwin). a deep cylindrical base ; attached 
to corals. Lower Devonian. 
Yoronula, Lam. Composed of six lateralia, with thin, deeply folded walls dividing 

Fic. 1368. 

Palaeocreusia devonica, Clarke. 
Embedded in Favosites. Corniferous ; 
Fic. 1367. 
Balanus pictus, Miinst. Miocene; Dischingen, Wiirtemberg. Le Roy, New York. 
the interior space into chambers which open at the lower side of the shell. Base 
membranous ; parasitic on whales. Pliocene to Recent. 
Chthamalus, Rauz (Euraphia, Conrad). Shell depressed, composed of six pieces. 
Base membranous. Cretaceous, Miocene, and Recent. 
Pachylasma, Darwin. Shell in the young with eight pieces, which afterwards hecome 
six, or by coalescence of the lateralia are apparently reduced to four. Base calcareous. 
Pliocene to Recent. 
Super-Order 5. MALACOSTRACA. Latreille. 
Ewerustacea having, in recent forms, a constant number (twenty or twenty-one) of 
segments. Cephalothorax composed of thirteen segments, and bearing the same number 
of appendages. Abdomen distinct, heptamerous (except in the Phyllocarida), the 
terminal joint being known as the telson. 
Under this head are included the orders Phyllocarida, Stomatopoda, Decapoda, 
and <Arthrostraca, all of which have fossil representatives. 
