nera, narrow
and elongated in form with terminal margins of end valves elevated,
belong to this group.

Suborder II. MESOPLACOPHORA, Pilsbry.--Insertion plates well developed
and slit.

Fam. 2. _Ischnochitonidae._--All the valves with slits, and the
inner layer well covered by the outer.

Subfam. 1. _Ischnochitoninae._--No shell-eyes: sutural laminae
separated; slits in the valves 1-7 do not correspond with the ribs
of the tegmentum. _Ischnochiton, Trachydermon, Chaetopleura,
Stenoplax, Stenoradsia_.

Subfam. 2. _Callochitoninae._ With shell-eyes and united sutural
laminae. _Callochiton laevis_, North Atlantic and Mediterranean.

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. Loven, 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.

The chief points in which the Aplacophora differ from