The Dentition of British Mollusca. 69 



glass blackened at the back, is to be placed inside at such an 

 angle as to reflect the light through the hole. The object 

 is then laid on a glass slide over the opening and cleaned with 

 a camel-hair brush and distilled water. 



Only a portion of the tongue is in use at any one time. 

 This is nearly flat and is held in its place by projections of the 

 membrane on either side. The posterior part descends ob- 

 liquely behind the mouth, and is formed into a cylinder by being 

 enclosed in a membranous tube, which peels off like the finger of 

 a glove turned inside out, and allows the whole of the lingual 

 ribbon to be displayed as a flat strap. If particles of tissue 

 adhere to it, they may be carefully removed by the brush, or 

 the curved needle. But being very delicate, the tongue is 

 often liable to be torn, if held meanwhile by a hard point ; for 

 this purpose a bristle is a very handy tool. The front of the 

 tongue is in some cases folded at its end, so that the part 

 most in use is at a short distance from the extremity. This 

 happens especially with the carnivorous species which bore 

 through the shells of their prey. The teeth on this portion are 

 frequently worn down and broken, and as it is essential to the 

 well-being of the animal to have good teeth, the reserve so 

 bountifully provided is brought gradually forward, the worn 

 part being at the same time absorbed. Thus a continually new 

 rasping surface is secured. Quite at the hinder end of the tongue 

 the teeth become rapidly imperfect and rudimental; but it 

 admits of doubt whether they are in the act of growing, since 

 the lingual band would appear to be originally prepared of such 

 a length as to last effective as long as its owner requires it. In 

 our periwinkle, the spare portion will be found beautifully coiled 

 up in the body of the animal on the right side. That of the com- 

 mon limpet passes backwards and downwards, doubling on itself 

 in its course, and is more than twice as long as the mollusk. 



The tongue itself is divided for convenience of description 

 into longitudinal areas, which are crossed by the rows of teeth. 

 Of the former there are five, distinguishable by the different 

 characters of the teeth they bear ; but they are not always 

 all present. The teeth are consequently named the median, the 

 lateral, and the uncini, although the latter are not necessarily 

 more hooked than the others. The areas bearing the uncini 

 have been called pleurce. Since each row is a repetition of all 

 the rest, the system of teeth admits of easy representation by 

 a numerical formula, in which, when the uncini are very nume- 

 rous, they are indicated by the sign oo (infinity), and the others 

 by the proper figure. Thus, oo. 5. 1. 5. oo, which represents 

 the system in the genus Trochus, signifies that each row con- 

 sists of one median, flanked on both sides by five lateral teeth, 

 and these again by a large number of uncini. When only 



