Subfam. 3. Callistoplacinae. No shell-eyes, slits in the valves 1-7 corresponding with the ribs of the tegmentum. Callistochiton (viviparous). Nuttalochiton.
Fam. 3. Mopaliidae. Each intermediate valve with a single slit; girdle hairy. Mopalia, Placiphorella, Plaxiphora, Placophoropsis.
Fam. 4. Acanthochitonidae. Valves immersed in the girdle, with small tegmentum. Acanthochiton (A. fascicularis, North Atlantic and Mediterranean). Spongiochiton, Katharina, Amicula, Cryptochiton (C. stelleri, arctic).
Fam. 5. Cryptoplacidae. Vermiform, with thick girdle and small valves; insertion and sutural plates strongly drawn forward, sharp and smooth. Cryptoplax, Choneplax.
Suborder III. Teleoplacophora, Pilsbry.—All the valves, or at least the seven anterior, with insertion plates cut into teeth by slits.
Fam. 6. Chitonidae. Characters of the suborder.
Subfam. 1. Chitoninae. No extra-pigmental eyes; insertion plates with pectinations between the fissures. Chiton, Eudoxochiton, Trachyodon, Radsia.
Subfam. 2. Toniciinae. Extra-pigmental shell-eyes. Tonicia, Acanthopleura, Enoplochiton, Onithochiton, Schizochiton, Lorica, Loricella, Liolophura.
Order 2.—Aplacophora, von Jhering.
Chaetoderma was first described by S. Lovén, in 1841, and was for a long time believed to be a Gephyrean worm. Neomenia, mentioned first by Michael Sars in 1868 under the name Solenopus, was afterwards included among the Opisthobranchs by J. Koren and D.C. Danielssen. C. Gegenbaur placed the two genera in a division of Vermes which he called Solenogastres.