166 NATURAL CLASSIFICATION OF THE GASTEROPODA. 



and I find that Jhering has observed the same structures in the 

 genus Peronia, which lives between tide-marks, thus confirming 

 Ehrenberg's view. 



3rd. I can vouch myself for the coexistence of true branchiae 

 within the pulmonary chamber in both SipJionaria and Amphibola, 

 having made drawings of them from the recent animals. 



The branchial characters of the Nudibranchs have been so 

 worked upon and variously represented by different writers, that 

 I have only for the present made two sections of them. The first, 

 or the Cryptobranchia, to include those forms which at best only 

 present a ciliated surface for respiration ; and the second, or Pha- 

 nerobranchia, those in which the branchial organs are plainly dis- 

 cernible. Perhaps a much more satisfactory guide to classifica- 

 tion will be found in the lingual and labial dentition, which, 

 though exceedingly perplexing to the student, will, 1 am quite 

 sure, be better understood when opportunities are more favourable 

 for direct comparison and legitimate deduction. 



There are, indeed, certain principles to be borne in mind when 

 we enter upon the study of the dentition of a family or larger 

 group exhibiting great diversity as to the number of the elemen- 

 tary parts and their particular form. The essential points to 

 know are the following : — 



The typical lingual dentition presents a central region, or 

 rhachis, and two lateral parts, or pleurce so-called. The dental 

 organs are disposed in transverse rows succeeding one another from 

 before backwards ; and the lingual membrane upon which they are 

 arranged is quite homogeneous and more or less corneous in con- 

 sistency, supported by the lingual cartilages, and forming the 

 floor and sides of the lingual sac, which latter usually projects 

 downwards and backwards below the oesophagus from the floor of 

 the mouth. 



When the lingual membrane is comparatively short and broad 

 and the teeth numerous and similar, the dentition assumes the 

 form of a pavement ; but, on the contrary, if the membrane is very 

 much longer than it is broad, and there is a marked distinction 

 between the rhachis and pleurse, it acquires the character of a 

 "ribbon" or " strap." 



It is curious to remark, and it bears largely on the soundness 

 of the primary division in the foregoing table of classification, 

 that the dentition is typically pavemental in the Monoscious and 

 ribbon-like in the Dioecious Gasteropoda ; and, with certain ex- 

 ceptions, even admitting of explanation, the auditory sacs contain 



