with a revision of the Limnmdce of Authors. 359 



is the type. In so small an animal it might readily be over- 

 looked or even be entirely absent. 



It is to be hoped that the numerous observers who are fur- 

 nished with microscopes will avail themselves of the riches 

 which lie hidden in every brook and pool. The dentition and 

 anatomy of our native species of P/njsa, Limncea, I'lanorbis, 

 as well as the Yiviparidm and the Strepomatidce are almost 

 entirely unknown. 



The ribbon may be most easily obtained by boiling the ani- 

 mal in a test tube over a spirit lamp, in a solution of caustic 

 potash. It can easily be found by pouring the contents of the 

 tube into a smooth white saucer, or a deep watch-glass. The 

 shell from which the animal was extracted should always be 

 preserved, and the ribbon mounted, when practicable. 



Great care is needed when inexperienced in such work, par- 

 ticularly when working with a low power, not to mistake the 

 outline of the base of insertion of the tooth for its cusp. The 

 latter is almost always transparent, and nearly invisible when 

 compared with the base, w r hich is usually dark yellow. 



A good plan is, when specimens are plenty, to break up the 

 ribbon after a close examination of it in the entire state. In 

 this way the outlines of the edges of the cusps are more easily 

 made out. Drawings, no matter how rude, should be made on 

 the instant, and repeated until the entire accuracy of the out- 

 lines is beyond dispute. Some assistance may be obtained in 

 such work from photography, which, uncorrected by a trained 

 eye, is, however, very liable to mislead. Tiie extreme ante- 

 rior teeth are usually much worn and broken by use, and the 

 extreme posterior ones are immature and not fully Bhaped. 

 The middle of the odontophore affords the best examples. 



The jaws are destructible in potash and should be examined 

 in situ, by placing the animal on a needle stuck into a cork, 

 which should be fixed on the stand of the microscope, and light 

 should be condensed by a bull's-eye lens on the part ot the 

 animal which it is desired to observe ; but not too strongly, or 

 it w T ill be burned or dried up. 



