l64 ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA 



diameter. Each contains a single strongly refracting globular otolithe 

 of about -y^^dth of an inch in diameter. In some cases this had a slow 

 movement of rotation upon its axis, fig. 6,7'. 



Now in regarding Firoloides and Atlanta, whose structure has just 

 been described, as illustrations of a typical form, the following circum- 

 stances appear to me to be of importance ; — 



1. The intestine is bent dorsal, or towards the side on which the 

 heart is placed. The visceral mass is situated below and behind the 

 posterior portion of the alimentary canal ; it may be called a post- 

 abdomen. 



2. Atlanta is prosobranchiate ; Firolo'idcs is neither opistho- 

 branchiate nor prosobranchiate. 



3. The foot consists of three parts, the propodium, mesopodium, 

 and metapodium, in Atlanta ; but of these the mesopodium disappears 

 in Firoloides, and the metapodium becomes very rudimentary. 



4. The auditory organs appear to be connected with the cephalic 

 ganglia. 



5. The animals are unisexual. 



III. Anatomy of Pteropoda. (Plate IV. [Plate 19]) 



The variation of form undergone by the members of this group is 

 perhaps greater than that which takes place in any other, except it 

 may be the Nudibranchiata, and hence their structure is particularl}' 

 instructive. 



Three very distinct modifications of the type present themselves 

 at first sight. The first is characterized by the non-development of 

 the mantle and the full development of the foot, ex. Piteunioderinon, 

 Clio, fig. I. 



The second, by the great development of the mantle, by its cavity 

 opening upon the ventral surface, and by the minuteness or absence of 

 the mesial portion of the foot, ex. Cleodora, fig. 4. 



The third resembles the second, but the mantle-cavity is placed 

 upon, or at any rate opens upon, the dorsal surface, ex. Spirialis, 

 Liviacina. 



I. It is very remarkable that Cuvier should not have recognized in 

 the " espece de menton," and the " deux petits levres "^ of Pncumo- 

 dermon, nor in the " deux tentacules triangulaires " of Clio} the homo- 

 logues of the foot of the Gasteropoda. In fact, it was on the strength 

 of their having no such appendage that he founded his new order of 



^ Mem. sur le Pneumoderme, p. 7. 

 ^ Mem. SUV le Clio, p. 6. 



