MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



121 



Immediately beneath the buccal cone is still another organ which is peculiar to the male It 

 was discovered by Van der Hoeven, and lias since been known as Van dkk Hoeven's organ. 

 It lies in a pocket formed by the ventral portion of the cephalic sheath ventrallv. and dorsally 

 by a fold connecting the ends of the horseshoe-shaped ridge which bears the superior labial lobes. 

 (Text-tig. 4. V.) The organ can not be seen until the cephalic sheath has been opened, so closely 

 does it lie under the buccal mass. Even then the observer can only notice that its anterior end 

 appears to be formed of small lobes and is bilaterally symmetrical. In the diagram (Text-tig. 

 i, V) it is represented, for the sake of convenience in drawing merely, as projecting at some dis- 



Tf.xt-fig. 4.— Diagram of the arrangement of the tentacles of the male nautilus, viewing them from in front. 



AS. antispadix: B, buccal mass; CS, cephalic sheath, composed of the fused sheaths of the digital tentacles; (>', preocular' tentacle; 

 O", postocular tentacle; S, spadix; SLL, superior labial lobe: V, Van der Hoeven's organ. 



tance from the buccal mass, which it does not actually do. Its shape is roughly outlined by the 

 dotted line. I hope to present sufficient evidence to be convincing that Van deb Hoeven's organ 

 is the homologue of the inferior labial lobes of the female. 



SUPERIOR LABIAL LOBES. 



The only considerable difference between the superior labial tentacles of the male and those 

 of the female is in their number. Eight tentacles are usually borne upon each superior labial 

 lobe of the male, while twelve is the usual number upon each of these lobes of the female. Less 

 than eight tentacles may occur upon each lobe of the male, although I have never found more 

 than this number. Two of my specimens presented only six tentacles upon each lobe; one speci- 

 men had six upon one lobe and seven upon the other. 



The superior labial tentacles of the male are slightly more robust than those of the female; 

 their sheaths are more completely separated in the male than in the female. Two of the tenta- 



