124 TELLINID. 
shells are, with few exceptions, thin, delicate, of small size, 
generally having the transverse measure double that of the 
vertical; the siphons of the animal are slender, usually sepa- 
rated to their bases, nearly as proportionately long as in the 
Pholades, and sparingly cirrhated at their terminations. We 
consider the posterior flexure of the shell of little distinctive 
value: all bivalves have it more or less. The most curious 
incident attached to some of the Tellimidan genera is, the 
amalgamation of each pair of branchiz into one large lamina, 
the upper plate being as it were thrown back and perma- 
nently fixed to the dorsal range, as in the Anatine—a com- 
munity of structure showing a close alliance between the two 
families. The genera and species of this group exhibit a 
greater departure from what are considered the typical poimts 
of configuration, than in most others; for example, the 
external and internal ligament and cartilage march together, 
as well as the double and single branchie; but, notwith- 
standing the discrepancies of structure, these aberrant genera 
and species have so much of the family character that 
they cannot well be placed elsewhere. It may hereafter be 
necessary to recast the Tellinide, and constitute some new 
families and genera; for the present, we shall distribute the 
different groups into sections embracing their aberrations and 
peculiarities in respect to the typical configuration, but we 
still hope to supply some useful rectifications. 
The animals of Tellina and Psammobia are all but identical ; 
a rigorous examination of both does not allow us to state an 
essential difference either in the internal or external organs ; 
their shells also scarcely vary ; the posterior flexure in Psam- 
mobia is almost as apparent as in Te/lina. We think the genus 
might have been dispensed with; it is probably retained by 
authors because it forms a break in a long list, if the exotics 
are included; but that is not a sound reason for constituting 
two genera without a difference. We think that an essential 
service is done to science in relieving it of an artificial genus ; 
therefore, in our method, Psammodia is merged in Tellina, 
which forms the first or typical section. As to the supposed 
difference in the lateral dentition of Te/lina and Psammobdia, 
