17 



visceral mass and extend over the edge of the pen to the mantle. However , 

 the chief connection is furnished by the pen , to whose upper surface the mantle 

 is attached, whereas the large siphonal, nuchal and cephalic retractors are attached 

 to its lower surface. 



The pallial cavity is very large and extends back nearly to the dorsal point 

 of the mantle. By the expansion and contraction of the mantle a large quantity 

 of water is drawn into and expelled from the pallial chamber. 



The two triangular fins are thin muscle-sheets and together, they form a 

 flat rhomboidal plate which is attached to the upper 

 surface of the mantle and extends from its dorsal 

 point over ^5 of its length. The longest side, the 

 base , of each fin is attached to the mantle and the 

 two free edges are of nearly equal length. The 

 dorsal angles of the two fins form together an acute 

 angle which ends in a rounded tip. The lateral 

 angle of each fin is obtuse and very much rounded. 

 The ventral ends of the fins are some distance apart 

 and each forms a small auricular lobe. The dorsal 

 points of the cartilages of the fins are firmly united 

 by strong masses of connective tissue, but at the 

 middle the fins are not closely connected and overlap 

 one another considerably. They are united by a 

 thin subcutaneous sheet of muscle , which extends 

 across their upper surfaces, as well as by the 

 muscles which bind them to the mantle. The long 

 attached base of the fin is underlaid and supported 

 by a strip of cartilage — the fin cartilage — which 

 is thin and broad except at its dorsal end where it 

 becomes a strong bar that curves downward and inward over the dorsal point 

 of the mantle. The fin has very thin free edges, and increases in thickness 

 gradually toward the base where it is attached to the cartilage at an angle of 

 about 30° (text figure 6). The fins are attached to the mantle by the follow- 

 ing three muscles. A strong muscular band arises from the lower surface 

 of the fin cartilage near its dorsal end and passes backward around the tip 

 of the mantle to the opposite cartilage. A broad thin sheet of muscle arises 

 from the upper surface of the mantle in front of the fins and is inserted upon 

 ' the upper surface of the bases of the fins. A second broad thin muscle arises 



