72 



J. D. MACDONALD ON METAMORPHOSIS OF GASTEROPODA. 



shell. Ample provision is made for tliis contingency by a dense epitlielial pavement, whicli 

 lines the cartilaginous shell, and which may possibly be homologous with that by which the 

 test is evolved in the Tunicata. As the chief agent in the development of the outer shell, it 

 may be termed the outer mantle ; and there is much reason to believe that it is persistent 

 in the adult Broionia and in many other cases. It is probably Avithin an outer mantle, 

 homologous with this, that the first rudiments of the shell are formed in the HelicidcB, 

 and the outer thickened rim of the mantle in the adult animals may be the remnant of 

 such a structure. Its coexistence with the true epipodia in Aplysia and other instances 

 precludes the notion that it may be altogether composed of these processes which have 

 coalesced over the back. These facts and considerations afford a simple explanation of 

 the development of internal shells in general, not even excluding those of the Cephalopoda, 

 into which the homologues of the retractor muscles of the Gasteropoda are fixed, although 

 the shells themselves in the two Orders are not generally admitted to be homologous, even 

 against the light which the scutellum of the slug is calculated to throw on the svibject. 



I must not omit to notice the capture of a minute 'Pleurohranclius in company with the 

 pelagic forms, off the Oato Reef. It was entirely naked, with broad ciliated head-lobes, 

 or festooned folds, apparently continuous with the epipodia. The large and beautiful 

 lalnal plates exceeding the dimensions of the double lingaial pavement of uncinate teeth, 

 and the auditory sacs (in this stage containing single but minute otoliths) lying wide 

 apart from the eye specks, at once distinguished it from Doris and other allied genera. 



Besides the numerous groups yet to be determined as fulfilling the conditions set forth 

 in this paper, I believe that I have already identified members of no less than six or seven 

 natural families, which may be distinguished by the simple characters given in the fol- 

 lowinsc Table. 



PELAGIC GASTEEOPODA (so called). 

 With strap-like dentition and otoliths in the ear-sacs. 



Shell 



& 



Operculum. 



Outer lip en- 

 tire. 



Operculum with a strong spine 



in front Strombid^. 



Nucleus minute, 

 not continuous 

 with the sub- 

 sequent lines of 

 growth Ranellidje 



Operculum <( 

 I simple. 



Nucleus large, 

 continuous with 

 the lines of 

 growth Naticid^ 



f Calyptr^id^ 



f Cancellated ?■! & 



Outer lip \ Cypr^id^. 



with claw- <| Double-keeled Muricid^. 



i^ like lobes. I 



I Smooth BucciNiD^. 



Ciliated 

 arms 6. 



)- Ciliated 

 arms 4. 



Ciliated 

 arms 4. 



I Dentition 

 ■^ septiserial. 



> Animal. 



Dentition 

 triserial. 



To these may be added Brownia, and those species which are included in a cartilaginous case. 



