414 NEW GENUS AND SOME NEW SPECIES 



undulations or slight elevations, which should always be no- 

 ticed, as they are highly characteristic. The situation of the 

 beaks, when peculiar, should have the student's attention. 

 They are sometimes almost medial, as in the U. irroratus, U. 

 circulus, U. lacrymosus, 8fc. ; while in the U. ellipsis, U. sca- 

 lenius, U. cylindrieus, Symphynota tenuissima*, 8fc. they are 

 almost terminal: this character, however, varies. (See ob- 

 servations on the teeth.) 



The margins or circumference should have our attention 

 in examining a specimen. The general form of the Naiades is 

 ovate, modified into rhomboidal, triangular, circular, and ellip- 

 tical ; but these forms in the same species will frequently vary, 

 and therefore must notbe entirely relied on. The U. siliquoi- 

 deus is generally subangular posteriorly, but it is sometimes 

 truncate, and the U. cariosus is found in the same way. We 

 find very few species that are constant in this character ; this 

 accounts for the many species created from the U. pictorum 

 in Europe, 



Muscular impressions. These are important, and should 

 always have our attention in examining a specimen. But 

 even this character is not infallible. It should be understood 

 that the animals of this family always possess two pairs of 

 muscles, used for locomotion, and placed near or in contact 

 with the two adductor muscles, used solely for closing the 

 valvesf. In the anterior margin these are generally sepa- 

 rate, in the posterior more generally confluent; but in the 

 same species we sometimes find individuals presenting two, 

 sometimes three, and sometimes four cicatrices, besides 

 those of the cavity of the beaks; and this depends in a 

 great measure on the thickness of the shell. If the spe- 

 cimen be ponderous, we often find the posterior mus- 

 cle of the foot attached to the side of the lamellar tooth 

 near to its termination ; if it be thin, although of the same 

 species, it will be found generally confluent or near to the 

 great posterior muscle. The cicatrices, made by the attach- 



* Herein described. 



t See my description of new Uniones in this volume. 



