the Lingual Dentition in the Gasterojpoda. 241 



pavement, as in lanthina or Scalariaj or in the pleurae of a 

 lingual ribbon, as in Atlanta and Garinaria, There are, how- 

 ever, mixed characters in the dentition of some genera; and 

 the subject requires much careful study to elicit all that may- 

 be deducible from it as a guide to classification. 



It would be very desirable to establish a fixed nomenclature 

 for the parts, and some uniform mode of description of the 

 lingual apparatus, so that the dental characters of any species 

 referred to the fixed types shall be definite and unequi- 

 vocal. 



I have already employed Prof. Huxley's excellent name 

 " odontophore " for the tooth-bearing membrane, with its ex- 

 panded alge in front embracing the tip of the tongue and being 

 continuous with the lining membrane at the sides of the oral 

 cavity, the posterior tubular portion of this organ, named 

 the lingual sac, carrying the teeth upon its floor, extending 

 backwards to the closed extremity containing the dental pulp 

 or formative matrix, and forwards over the tip to the frsenum 

 of the tongue. The upper wall of the lingual sac terminates 

 anteriorly in a crescentic fold, by which it becomes continuous 

 with the lining of the oesophagus. The fore part and body of 

 the tongue proper is supported by lateral cartilages wrapped 

 together by muscle and ligament at the mesial line, and often 

 having smaller supplementary pieces moveably articulated in 

 front, as in the Turbos and Nerites. The cartilages in the 

 carnivorous families in particular conjointly form a grooved 

 surface, over which the odontophore glides when in action. 

 The common dental area or the space occupied by the teeth is 

 usually divided into three lesser longitudinal areas, a central 

 and two lateral, commonly known as rachis and pleurae. The 

 latter name may be retained ; but the " central dental area," 

 though longer, is preferable to " rachis," which is not suffi- 

 ciently definite. 



The central area usually presents a median series of dental 

 plates, either alone or with one or more lateral series. The 

 median series, however, is often suppressed. The plem'se may 

 present one, two, three, or many longitudinal rows of teeth ; 

 and these are numbered, from within outwards, first, second, 

 third, &c. 



With what has been already said of the basal plates and 

 dental processes connected with them, this brief anatomical 

 sketch will answer all practical purposes. The special types 

 of dentition will be noticed when the groups or families which 

 they characterize come under consideration. 



Having completed the foregoing introductory remarks, I 



