18 



EAST COAST MARINE SHELLS 



teeth are usually normal, those in front 

 often much worn by use, while at the oppo- 

 site end are In an undeveloped state. 



The radula is very narrow in Lit- 

 torina and several times the length of the 

 entire animal. It is entirely absent in 

 certain families including the Corallio- 

 philidae whose members live upon coral and 

 are nourished by their exudations. In par- 

 asitical mollusks, as for example those 

 living upon sea urchins, the radula has de- 

 generated on account of disuse. Another 

 genus, Melanella, with similar habits lacks 

 both radula and Jaw. 



Some of the peculiarities worthy of 

 mention explain certain irregularities. For 

 example, young examples are not always like 

 their parents. In some adults the radula 

 is absent but present in juvenile speci- 

 mens of the same species (Harpa, a Pacific 

 shell, for example) . In Voluta the later- 

 al teeth often are lost in the adult form 

 but the young exhibit normal ribbons. 



Like the longitudinal rows of col- 

 or in a length of ribbon, with a central 

 stripe, every band of color is duplicated 

 in the same relative position on the oppo- 

 site side. 



The radula may readily be removed 

 from large mollusks by cutting near the 

 mouth but with small or minute species a 

 special process is necessary. The entire 

 animal, fresh or dried, is placed in a test 

 tube containing nearly a tablespoon of 

 caustic potash which has first been allowed 

 to liquefy in the air. It should boll 

 gently over a flame but the contents not 

 allowed to boil over. The material should 

 not be permitted to lodge upon the side of 

 the tube. When the solids are fully dis- 

 solved the contents are poured out quickly 

 upon a watch glass which may be stirred 

 with a rotary motion revealing the radula 

 in the center. A piece of white paper un- 

 der the glass will make the search easier. 

 The curved and elongated radula, unless mi- 

 croscopic, may be removed with a needle 

 point. Placed in a drop of water it is 

 ready for examination under a cover glass. 

 A low objective in the microscope or a good 

 magnifying glass will suffice. 



On account of their transparency 

 many of the radulae, especially among the 

 small species, require staining which will 

 not injure the delicate teeth. Shields 

 Warren recommends for the marine forms a 

 saturate aqueous solution of potassium bi- 



chromate which may be allowed to act for 

 from five to fifteen minutes. An indefi- 

 nite period of immersion does no injury. 

 Land and fresh-water species cannot be 

 treated in the same manner and for these a 

 5-per cent solution of chromic acid works 

 well. The radula is placed on a slide, 

 covered with four or five drops of the acid, 

 and heated until the acid precipitates at 

 the edge of the drop. If overheated the 

 teeth may become separated from the lingual 

 ribbon. A thorough washing is necessary 

 after either process. The ribbons may then 

 be mounted in balsam which is the most sat- 

 isfactory medium. Glycerine Jelly makes a 

 suitable temporary mounting material but 

 the staining process should be used first. 



The MEDIAN or RACHIDIAN tooth of 

 the radula is the central one. 



The LATERAL or ADMEDIAN teeth lie 

 between the other two sets and are less 

 numerous but usually larger and more varia- 

 ble. When one of these is more prominent 

 it is called the MAJOR LATERAL, and others 

 are the MINOR LATERALS. 



The MARGINAL TEETH or UNCINI, near 

 the margin, differ sufficiently in charac- 

 ter to separate them from the others. They 

 are small, simple, and very much alike. 



The radula is bilaterally symmetri- 

 cal. Any series of the teeth may be ab- 

 sent, the median often being absent in cer- 

 tain species. The cusps, when they are 

 present, are very brittle and break readi- 

 ly under the pressure of a cover glass. Ab- 

 normal or deformed radulae occur and from 

 these the student should not draw incor- 

 rect conclusions. It is advisable to pre- 

 pare several specimens simultaneously and 

 at least one perfect example will likely 

 result. 



The DENTAL FORMULA is used to ex- 

 press the number and situation of the teeth. 

 For example: 



8 



3:1:3 



8 



indicates one median tooth in the center; 

 three lateral teeth adjoining; eight uncln- 

 al teeth upon the outside. 



l^'hen any of the series is absent a 

 cipher is employed to indicate this. Thus: 



shows that the median tooth is absent, that 

 there are three lateral and three uncinal 



