814 DESCRIPTION OF PLATES. 



Between the bases of the cephalic tentacles occurs a free flap of the integument 

 resembling a nuchal membrane (Fig. 5). x 3. 



N.B. — No portion of the mantle appears in either figure. 



Fig. 6. iV. pomjnlius juv. Dorsal view of a portion of the mantle-region of the same 

 specimen as PI. LXXVII. fig. 4. 



d.p.i. The line of insertion of the dorsal lobe of the mantle into the body-wall. 



s.m. Shell-muscle. 



an. Annulus. 



s.a. Septal aponeurosis. 



The succeeding figures, unless otherwise stated, relate indifferently to N. pompiliiis and 



N. niacromphalus. 



Fig. 7. Piece of the posterior region of the mantle viewed from the inner or coelomic 

 surface, to illustrate the connections of the siphuncle. The piece comprises the entire dorso- 

 ventral extent of the insertion of the genito intestinal ligament into the hinder wall of the 

 perivisceral coelom. 



g.l. Genito-intestinal ligament (cut across). 



p.a. Posterior pallial artery. 



p.v. Posterior pallial vein. 



s.a. Siphuncular artery (sinistral). 



r.s. Radical sinus of the siphuncle (pallio-siphonal sinus). 



s. Siphuncle. 



Fig. 8. Similar piece from another specimen. The posterior pallial vein and pallio-siphonal 

 sinus have been laid open and a bristle passed into the siphuncular vein (axial cavity of 

 siphuncle). The siphuncular artery (dextral) is seen in the floor of the pallio-siphonal sinus. 



Fig. 9. Portion of a bisected shell of T. pompilius to illustrate the relation between 

 the growth-lines of the shell (shell-lines), the concentric muscle-lines and the posterior border 

 of the actual muscle-scar and annulus. An asterisk is placed opposite to the linea limitans 

 of the muscular impression. The last-formed septum is thin and fragile, thickening not having 

 yet taken place. 



PLATE LXXVII. 



Fig. 1. Frontal view of the fundus of the animal-chamber of a shell of N. pompilius. 



1. Annular impression. 



2. Septal suture or impression of the septal aponeurosis of the mantle. 



3. Muscle-scar. 



4. Border of the nacreous layer deposited by the dorsal lobe of the mantle over the 

 black varnish which covers the normally banded periostracum of the anfractus of the shell. 



Fig. 2. Specimen of female N. macromphalus with spermatophore {sp.) protruding from 

 the oral sheath. 



Fig. 3. N. macromphalus adhering to the substratum in a vertical position by means 

 of its tentacles. More usually its position, when attached to any surface, is more or less 

 horizontal, with the adhesive tentacles spreading about in all directions. 



