US THE NAUTILUS. 



character. In the normal hinge the (single) lateral teeth of the left 

 valve are higher than those of the right one, usually projecting above 

 the level of the valve-edge. The reversed teeth are so in the right 

 valve. Reversion in one or other degree was seen in hundreds of 

 specimens of the Sphaeria s. str. : simile, striatinum, stamineum, (v.) 

 emarginatum and other forms, jlavum, fabale, nobilr, primeanum, 

 and in lots from some localities in 20 to 30 per cent, of all specimens. 

 Might it be inferred, from the great instability of the hinge characters, 

 as well as the almost endless variability in shape, size and striation of 

 some species, that the whole group is of a recent geological age, with 

 the features not fully established? Has any such variability been 

 noticed in Corbicula, etc., or in the marine Cardiacea? 



In Sphaeria -rhomboid e ion, occidental, corneum, etc., reversion 

 seems to be rare; and so in Caltjculina. It has been noticed in Pisidium 

 virginic um (three specimens, cardinal and anterior laterals), abditum 

 (totally and partly), and politum (one specimen, totally reversed). These 

 were the only instances noticed among many hundred, probably thou- 

 sands, of Pisidia examined for the hinge characters. 



3. Ridges on the beaks of some Pisidia. Ridges (or appendages) 

 are known to be present on the beaks of a number of species, such as 

 supinum, henslowamon, compression, fallax, cruciatum, punc- 

 tatum, ferruyineion, and for some of them they have been described 

 as characteristic. Of the North American species they have been seen 

 wanting in P. compression, fallax and punctatum, usually in forms 

 which are characterized also by other peculiar features, and must be 

 regarded as varieties. But sometimes all possible intergradations may 

 be seen among specimens from one locality and ranging under the same 

 ''form" or variety. In P. eructation the singularly shaped ridges 

 have been found absolutely constant so far. 



On the other hand, beaks with their tops more or less flattened, and 

 with slight indications of ridges, may be seen in species where they are 

 usually rounded, as in P. variabile, abdititm, splendidulum. Among 

 lots of the latter species, from Aroostook county, Maine, specimens 

 were found with very strong ridges, just as in P. ferritgineion, and 

 they would have to be regarded as representing a widely distinct specie's, 

 if it were not for intermediate forms. 



This is one well marked example of the often perplexing variability 

 of those small mussels, and strongly urges the student not to rely on 

 one or other ever-so-striking feature for the distinction of species, but to 

 carefully consider the ensemble of all the different characters, all of 



