XIII DECAPODA 389 



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 Trachyteit- 

 this (fossil only). 



C. Chondrophora. — Shell (gla- 



dius or pen) long, chitinous. 



(a) Myopsidae:'^ cornea entire. ^ ^,^ ^ ~ ',. ^ ,^ , 

 \yi^r^ ' Fig. 249.— Hectocotylised arm (/i. a.) 



species mostly sub-littoral. of Sepia officinalis L., shown in 



Fam. 1. Sepiolidae.— Fins large, ^^"*?^^* *« «°^ «^^^^ °5f °^^>^ 



■^ ° ' sessile arms; m, mouth; p, 



dorso-lateral ; tentacular arms retrac- pocket into which the tentacu- 



tile; two first dorsal arms in the lar arm is retracted. 



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 ; Bossia, dorsal mantle supported 

 by a ridge, arms with never more than four rows of acetabula ; 

 Inioteuthis, Stoloteuthis, Nectoteuthis^ and Promaehoteuthis. 



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. 



Fam. 3. Idiosepiidae. — Fins very small, terminal; fourth 

 pair of arms in the male hectocotylised, bare of suckers. 



The only genus, Idiosepion, with a single species (/. pyg- 

 maeum Stp.) is from the Indian Ocean, and is the smallest 

 known Cephalopod, measuring only about 15 mm. in length. 



Fam. 4. Loliginidae. — Body rather long, fins varying in size, 

 tentacular arms partially retractile, gladius as long as the back, 

 pointed in front, shaft keeled on the ventral side. — World-wide. 



Loligo proper has a pointed body with triangular posterior 

 fins united behind; sessile arms with two rows of acetabula, 

 ^^uuecw, close the eyes ; yi|/is, sight ; contrasted with Oigopsidae (oi^w, open). 



