278 SHELLS AND SHELL-FISH. PART I. 



fortunately, the group^ although not so extensive as the 

 firstj is yet rich in subordinate types ; we shall, there- 

 fore make some approximation to the natural series of 

 the species, by looking to the following table, wherein 

 each subordinate type is indicated by its most prominent 

 example. 



Analogies of the Sub-generic Ti/pes of My sc a. 



rr, 4 1- Genera of 



^yP''' Analogies. Unionidcc. 



Mv«caovata f Both cardinal teeth presentj-^hort, 7 ^1*10 



Myscaovata. ^ and jaggedjoue in the left valve, j ^^''^• 



^„ . i-T CTwo nearly equal cardinal teeth 7 «..„,» 



marginahs. J in the left valve. j "^^'^• 



nasufa I" ^"^^"^ cardinal tooth nearly erect, ) TR,nivA 



nasuta. ^ pointed, somewhat conical. j iridina. 



Nilotira ("Oval; hinge sub-connate; poste- 7 Anodox 



JNllotica. ^ rior angle winged. j anodox. 



f. .,. ' f Hinge sub-connate: eardinalteeth 7 a,.,,,^„^„ 

 fragihs. I infperfect. j ALAbMODON. 



(261.) Among the longest species of Mysca generally 

 known are our two British species, ovata and pictorum, 

 which may consequently be considered typical; and these 

 also have the most crenated cardinal teeth. Following 

 these we have a type from India, represented by the mar- 

 ginalis of Lamarck, where the outer cardinal tooth is long 

 and slender, as in Hyria; and the other similarly formed, 

 but much shorter : these shells are also remarkable for 

 having two distinct cardinal teeth, almost alike, in the 

 other or left hand valve, — a structure which is quite 

 different from that of ovata, and indeed of all other 

 Unionidce. Let the malacologist examine the Hyria 

 avicularia and the Unio marginata of Lamarck ; and 

 so far as regards the cardinal teeth, he will find they are 

 formed precisely on the same model : nothing, there- 

 fore, can be more perfect.* Marginalis agrees also with 

 Iridina, in being, without exception, the most iridescent 



• We even question whether this may not be a relation of affinity, and 

 thus Lamarck's marginata may be the anodontine type in Hyria. 



