IV PREFACE. 



subject, and to have it exaAiined into by those conversant with 

 it, I give a general outline of some of the changes which I 

 think, in the present state of our knowledge of this family, might 

 be made with advantage. 



In the first place I propose to separate the American recent 

 species of the genus Corbicula from the species of this genus in- 

 habiting other regions, for the reason that in the American species 

 the palleal impression is terminated in a sinus, whereas in the 

 foreign species it is simple. This is a character derived from the 

 soft parts, though the knowledge of it is conveyed to us by an 

 examination of the shell. The same distinctions exist between 

 the American recent species of the genus Cyrena and the foreign 

 species of that genus; it may, therefore, likewise be advisable to 

 constitute a separate genus for the American recent species of 

 Cyrena. 



In connection with this subject I may mention that three fossil 

 species of Corbicula from the Paris basin, which I have had 

 under examination, the Corbicula cuneiformis, antiqua andfor- 

 besii, have the palleal impression terminated in a sinus as in the 

 American species. It may be necessary to remove these species 

 from the old genus Corbicula and place them in the genus pro- 

 posed for the American recent species of Corbicula. 



The genus Sphaerium w^ould I think be benefited by being 

 divided into four genera, as follows : — 



1. Shell solid, strife deep, beaks rounded ; example, Sph. sulcatum ; 



Sph. solidum of Europe. 



2. Shell somewhat solid, striae light, beaks rounded ; example, Sph. 



rhomboideum ; Sph. corneum of Europe. 



3. Shell delicate and pellucid, striae not perceptible, beaks calycnlate ; 



example, Sph. parlumeium ; Sph. lacustre of Europe. 



4. Shell very small, delicate, transverse, striae very light, beaks calycn- 



late; example, Sph. bahie}ise; Sph. africanum of South Africa. 



These last proposed genera are based upon characters drawn 

 from the shell alone; an examination of the soft parts of the first 

 three proposed subdivisions is however now being made by my 

 friend, jNIr. Edward S. Morse, of Gorham, Maine. With regard to 

 the fourth proposed subdivision, which I am very confident ought 

 to constitute a separate genus, I regret to be obliged to state, that 

 as yet, all my efforts to obtain alcoholic specimens of any of the 

 species have been unsuccessful. The shell of the species of this 



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