6 



4. fragilis, Lam. 7. petalifera, Rang. 10. truncata, Rang. 



5. gigas, Rang. 8. Rumphii, Guv. 11. unguifera, iJ. 

 G. Hassettii, id. 9. Teremidi, 7t««^/. 



Figures. 



Plate 2/. Animal, showing its front ear-like pair of tentacles and its 

 structure of overlapping folds, concealing the branchial lid which 

 contains the shell. 



PL 22. Pig. 127. Shell, showing the thickened incurved margin of the 

 hinder extremity. 



Family 3. SEMIPHYLLIDIANA. 



Shell ; horny or shelly, more or less imbedded in the upper mr- 

 face of the animal for the protection of the viscera. 



The genera embraced in this family are not very closely allied, but they 

 agree in having an internal shell secreted, not over a branchial cavity, for 

 the branchiae are free, but over the viscera. 



Pleurobranchus. Umbrella. 



Genus 1. PLEUROBRANCHUS, Cuvier. 



Animal; head with two grooved tentacles, the eyes at their ex- 

 ternal bases ; mouth covered with a more or less broad veil ; 

 body equally overlapped by the mantle and by the foot ; mantle 

 sometimes enclosing a delicate membranaceous shell ; branchiae 

 feather-like, free. 



Shell ; an internal, flexible, membranaceous shield, oblong-scpiare, 

 rather convex towards the margin, where a kind of subspiral 

 apex is formed. 



Pleurobranchus is only known to the conchologist by a thin membra- 

 naceous concavely-flattened shield, secreted within the hinder portion of the 

 mantle. The mollusk from which this is extracted is a broadly-ovate, 

 fleshy mass, presenting great variety of character in different species, but 

 having the branchiae projecting from the right side in the form of an 

 elaborately-branched plume. Of the following species, two, P. plumula 

 and membranaceus, are found at low water on our own shores. 



