118 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 
specimensof the Sphaeria s. str.: simile, striatinum, stamineum, (v.) 
emarginatum and other forms, flavum, fabale, nobile, primeanums, 
and in lots from some localities in 20 to 30 per cent. of all specimens. 
Might it be inferred, trom 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, ete., or in the marine Cardiacea? 
In Sphaeria rhomboideum, occidentale, corneum, etc., reversion 
seems to be rare; and so in Calyculina. It has been noticed in Pesidium 
- virginicum (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, henslowanum, compressum, fallax, cruciatum, punc- 
tatum, ferrugineum, and for some of them they have been described 
as characteristic. Of the North American species they have been seen 
wanting in P. compressum, fallax and punctatum, usuaily 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. cruciatum 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, abditum, splendidulum. Among 
lots of the latter species, from Aroostook county, Maine, specimens 
were found with very strong ridges, just as in P. ferrugineum, and 
they would have to be regarded as representing a widely distinct species, 
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 
