GASTEROPODES. 281 



in general. When adhering to the surface of the vessels whereon speci- 

 mens repose, it should be removed. The Abb6 Dicquemare remarks, 

 that an organ beneath the posterior extremity is the source of this 

 mucous matter. 



Some of the race of the Doris are more languid than others, but all 

 are extremely sluggish. The smaller swim supine, but they can be 

 seldom incited to motion. 



From the great diversity of size and colour, from the scantiness of 

 the branchiae, their dimensions and appearance, also from the indentations 

 of the flap, sometimes seeming an entire piece, sometimes cleft almost in 

 two down the middle, I was long induced to contemplate the subdivision 

 of the Tritonia Hombcrgii into distinct species. But after seeing many 

 of all different dimensions, colours, and proportion, extending between 

 four lines and eight inches, I am not prepared to deny the identity of 

 the whole as belonging to a single species. Progressive evolution pro- 

 duces an incredible change. 



PLATE XXXVIII. 



FIG. I. Tritonia [Doris] Hombergli, adult, back. 



2. Belly. 



3. Corniculum of an adult. 



4. Branchiae. 



5. Younger specimen with few branchiae, back. 



6. The same, profile. 



7. Corniculum of the same, enlarged. 



8. One of the branchise of the same, enlarged. 



9. Minute specimen. 



10. The same, enlarged. 



11. Spawn of fig. 5. 



1 2. Portion of spawn, enlarged. 



13. Another deposit of spawn. 



14. Nautiline from the spawn, magnified. 



15. Nautiline from the spawn, magnified. 



16. Nautiline from the spawn, magnified. 



17. Nautiline from the spawn, magnified. 



18. Empty shells of the Nautiline, enlarged. 



2 N 



