288 Notes on Lingual Dentition of Mollusca. 



transverse rows, but there are in some instances five beak-like 

 denticles on the fourth lateral. It is difficult to follow this tooth 

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

 instances it is very much more expanded than shown in the 

 figure, resembling a gull's wing. The first beak-like denticle 

 seems to be on the same plane as the upper portion of the 

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

 tion ; this is shown in the figure by the line running parallel to 

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

 seems often to be recurved. 



The fifth lateral is with much difficulty found under the 

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

 is crowded into the excavation in the fourth lateral. It seems 

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

 cleaned 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 photo- 

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

 Powel that we are able to illustrate it satisfactorily. 



Fig. 8, 6, shows the central and one-half of one transverse row 

 of the laterals, with two uncini only. The balance of the uncini 

 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, Part III., fig. 216.) 



Fig. 8, a, represents the third lateral, which is not well shown 

 in its crowded position, as in h. 



On p. 108 of Land and Fresh-water Shells of North America, 

 Part III., a fac-simile is given of Troschel's figure of the lingual 

 dentition of the other species of the United States, Ilelicina orhi- 

 culata. A comparison of the two figures will show that the 

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



The specimen from which the membrane was extracted was 

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

 of its station : — 



