Fam. Sepiidae.—Eyes with cornea complete, body oval, fins narrow, lateral, as long as the body, generally united behind; sessile arms short, tentacular arms long, acetabula generally in four rows, fourth left arm in the male hectocotylised near the base (Fig. [249]).—World-wide.
The sepion or ‘cuttle-bone’ runs the whole length and width of the body. In Sepia it is very thick in front, while the posterior ventral end is concave and terminated by a prominent spine, the rostrum or mucro which points downwards. The whole shell is surrounded by a thin chitinous margin, which forms a lateral expansion. Other genera are Sepiella, Hemisepius, and Trachyteuthis (fossil only).
C. Chondrophora.—Shell (gladius or pen) long, chitinous.
(a) Myopsidae:[400] cornea entire, species mostly sub-littoral.
Fam. 1. Sepiolidae.—Fins large, dorso-lateral; tentacular arms retractile; two first dorsal arms in the male hectocotylised; gladius narrow, half as long as the body.—World-wide.
Principal genera: Sepiola, dorsal mantle connected with the head by a broad cervical band, ventral mantle with the funnel by a ridge fitting into a groove; Rossia, dorsal mantle supported by a ridge, arms with never more than four rows of acetabula; Inioteuthis, Stoloteuthis, Nectoteuthis, and Promachoteuthis.
Fam. 2. Sepiadariidae.—Fins not as long as the body, mantle united to the head on the dorsal side, fourth left arm in the male hectocotylised; no gladius. Principal genera, Sepiadarium, Sepioloidea.—Chiefly Pacific Ocean.
Fig. 249.—Hectocotylised arm (h.a.) of Sepia officinalis L., shown in contrast to one of the ordinary sessile arms; m, mouth; p, pocket into which the tentacular arm is retracted.
Fam. 3. Idiosepiidae.—Fins very small, terminal; fourth pair of arms in the male hectocotylised, bare of suckers.