288 V Lingual Dentition of MoUusca. 



trail . but there are in some instances live beak -like 



denl i the fourth lateral. It is difficult to follow this tooth 



behind the crowded uncini, but we are confident that in some 



es it is very much more expanded than shown in the 



hling a gull's wing. The first beak-like denticle 



on the same plane as the upper portion of the 



th; the other three are on the same plane as the* lower por- 



: this is shown in the figure by the line running parallel to 



upper edge of the tooth. The apex of the first denticle 



3 ofl i to be recurvt d. 



The fifth lateral is with much difficulty found under the 



microscope. It is on a different plane from the other teeth, and 



srowded into the excavation in the fourth lateral. It seems 



i wholly filled up with foreign matter, not being as readily 



Hied as the other teeth, even in a solution of potash. 



The whole lingual is a very difficult study, and requires 



numerous views to bring out the details of its structure by pho.to- 



phy. It is owing only to the. untiring perseverance of Mr. 



Powel that we are able to illustrate it satisfactorily. 



Fig. 8, b } shows the central and one-half of one transverse row 

 of the laterals, with two uneini only. The balance of the unci tti 

 Curve rapidly outwards and downwards, giving to the entire 



transverse section of the lingual membrane the usual strongly 

 arched outline. (See Land and Fresh-water Shells of North 

 America, Pari 1 1 1., fig. 216. i 



I •. 8, </. represeuta the third lateral, which is not well shown 



in its crowded position, as in /'. 

 < »n p. 103 of Land and Fresh-water Shells of North America, 

 : [II., a iae simile is given of Troschel's figure i if the lingual 

 dentition of the other species of the I fa i ted States, //> licina orbi- 

 \ comparison of the two figures will show that the 

 differ in their lingual dentition as widely as in their shells. 

 .men from which the membrane was extracted w 

 id living by Mr. E. R. Leland, who gives the following notes 

 station : 



