385 



forma in which the sexes are either separate, or combined in the in- 

 dividuals, no other characters can be of greater importance in esta- 

 blishing primary divisions. As a means of further subdivision ac- 

 cording to natural affinities, he suggests distinctive characters deriva- 

 ble from the auditory sacs and concretions, and from the oral, lingual 

 and gastric dental organs. 



In Mollusca, as in Fishes, the auditory concretions present them- 

 selves in one of two forms, viz. solitary lapilli, usually named otolithes, 

 or groups of small granules of a rounded oval or irregular shape, 

 which have been designated by the term otoconia. The lingual 

 teeth are either set together on a short and broad lingual membrane, 

 and form what the author calls a lingual pavement, or on a narrow 

 longitudinal band termed lingual ribbon or strap. The latter usually 

 consists of a median rachis flanked by two lateral portions or pleurae ; 

 but in some cases the rachis, and in others the pleurae are absent, and 

 the number of longitudinal rows of teeth in these divisions may also 

 differ in different genera. The fore part of the lingual membrane is 

 .supported by cartilage, so curved and fashioned as to receive the 

 lingual sac behind and form a basis to the tongue itself projecting in 

 front. This lingual cartilage may consist of a single piece thinned 

 in the middle line, or of two or four distinct pieces, similarly arranged 

 and wrapped together by muscle and ligament. The oral dental 

 organs or labial plates are disposed either horizontally or laterally. 

 In the former case a single plate may occupy the upper lip, or there 

 may be two guarding the aperture of the mouth, and corresponding 

 with both upper and lower lip, but the lateral plates are always in 

 pairs. Gastric teeth occur in the Aplysiadse and Bullidse. 



After pointing out further differences in the form and arrangement 

 of the dental apparatus in different genera, the author thus describes 

 the mode of development of the lingual teeth. " The lingual sac at 

 first appears as a little csecal process appended to the inferior part of 

 the oesophagus, where it joins the oral cavity. In the median line of 

 the floor of this sacculus, a few minute plates disposed in a longitudi- 

 nal row form the rudiment of the future rachis, and the progress of 

 their development may be distinctly traced on examining them, seria- 

 tim, from before backwards, in which direction, as their growth ad- 

 vances, they acquire a more perfect form. The internal row of pleural 

 plates now makes its appearance, their development proceeding in a 

 similar way ; and after this follow the others according to their posi- 



