Ventral view of head (A–B), subopercular region of left side (D–F), and subopercle from right side in medial view (G–J) in members of <i>Acyrtus</i> and <i>Arcos</i>.

<p>A, <i>Acyrtus lanthanum</i>, new species, holotype, FMNH 84325, 14.0 mm SL. B, <i>Acyrtus artius</i>, ANSP 123658, 15.2 mm SL. C, <i>Arcos nudus</i>, ANSP 115602, 25.6 mm SL. D, Close up of subopercular region of left side in A. E, Close up of subopercular region of left side in B. F, Close up of subopercular region of left side in C. G, <i>Acyrtus lanthanum,</i> new species, ANSP 106336, 20.4 mm SL. H, <i>Acyrtus rubiginosus</i>, UF 149202, 20.0 mm SL. I, <i>Acyrtus artius</i>, ANSP 94757, 24.2 mm SL. J, <i>Arcos nudus</i>, ANSP 142945, 27.0 mm SL. White asterisks indicate posteriormost tip of subopercular spine in D–E. Abbreviations: BR, branchiostegal ray; GSop, groove in subopercle; Sop, subopercle; VGS, venom gland cells. Scale bars equal to 1 mm (A–C, F) or 400 µm (D–E, G–J).