124 TELLINIDvE. 



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 Tellinidan genera is, the 

 amalgamation of each pair of branchiae into one large lamina, 

 the upper plate being as it were thrown back and perma- 

 nently fixed to the dorsal range, as in the Anatinte 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 points 

 of configuration, than in most others; for example, the 

 external and internal ligament and cartilage march together, 

 as well as the double and single branchiae ; 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 Tellinidce, 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 Tellina. 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, Psammobia is merged in Tellina, 

 which forms the first or typical section. As to the supposed 

 difference in the lateral dentition of Tellina and Psammobia, 



