Fig. 4. — Schematic plan views and transverse sections of grades of skeletal organization from the most primitive sessile barnacles (A, Brachylepadomorpha) through the asymmetrical sessile barnacles (B-H, Verrucomorpha). Of the eight verrucid genera currently recognized (Young 1998) only the four most representative ones (Altiverruca, Newmaniverruca, Metaverruca and Verruca s.s.) are depicted here. Right sides are to the viewers right and rostral ends are downward in plan views or coming out of the page in transverse sections. The transverse sections are slightly rostral of the rostro-carinal gap or suture. While there are numerous whorls of imbricating plates standing in tiers four plates high in brachylepadomorphs, and on the movable side of neoverrucids, only those of the R-C gap are labeled. Abbreviations: C, R, carina and rostrum; cl1-4, l1-4, rl1-4, carinolateral, lateral and rostrolateral tiers of imbricating plates respectively; cl, rl, the pair of imbricating plates in proverrucids; FS, FT, fixed scutum and tergum; hl, hinge line; L, median latus (completely lost in proverrucids and verrucids); m, myophore; MS, MT, movable scutum and tergum; sam, scutal adductor muscle; S, T, normal terga and scuta in brachylepadomorphs; dashed horizontal lines: membranous basis (transverse sections F-H after Young 1998).