31 



Cephalopods" has been made use of in this Memoir ; and to this group most of 

 the characters of the class as given by the immortal Cuvier in his Regne Animal, 

 exclusively appertain. 



In each lateral pair the branchiae are of unequal sizes, the larger one being 

 situated below, and on the outer side of the smaller. The larger branchia 

 {p. pi. 5. & 6.) is about one inch and two thirds in length, and two thirds 

 of an inch in breadth, and has forty-eight laminae on either side ; the smaller 

 branchia {q. pi. 5. & 6.) is about one third less than the preceding, and has 

 thirty-six laminae on either side. These laminae are disposed alternately, are 

 themselves composed of smaller transverse laminae, which are again similarly 

 subdivided (fig. 2. pi. 6.), the whole being connected together, and forming the 

 same tripinnatifid structure as in the Dibranchiate Cephalopods. The chief 

 differences consist in the branchial laminae of Nautilus being more closely 

 set upon the central muscular stem, and being more extended in the lateral 

 direction, so that the branchia is compressed from before backwards, and not 

 from side to side. Moreover, in Octopus and Loligo the branchiae are connected 

 to the inner surface of the sac by a membrane extending their entire length ; 

 but in Nautilus they hang freely in the pallial cavity, being attached only at 

 their bases ; in Sepia the extremities of the branchiae are unattached ; — an in- 

 teresting circumstance in connexion with the nearer affinity to Spirula and 

 Nautilus, which this genus already manifests in the laminated shell developed 

 within its mantle. The affinity of Sepia to Nautilus is also indicated by another 

 particular in its anatomy, viz. the organ*, hitherto considered anomalous, that 

 is appended to the branchial ventricle, but which appears to exhibit the rudi- 

 mentary condition of the accessory branchia, which we find to attain its full 

 development in Nautilus. 



The four branchiae of Nautilus receive the venous blood principally by four 

 vessels (5. 5. pi. 5. & 6.), which are continued from the central venous sinus, 

 arising from it on each side by a single trunk (4. 4. pi. 5. & 6.), which quickly 

 divides, and then proceeding to their respective gills, without being joined by 

 any other vessel, and without the interposition of any ventricle or branchial 

 heart as in the higher Cephalopods. 



In this course, however, they have attached to them clusters of glandular 



* See Home's Comp. Anat. iv. pi. 44. 45. fig. 9. 



