52 PROCEEDINGS OP THE ACADEMY OF 



Treble lateral tooth in the left, and double in the right valve. 

 Unio corrugatus. Lam., Bengal. 



Treble lateral tooth in the left, and partly treble in the right valve. 

 JJnio gibbosus, Barnes, Fox River, Wisconsin. 

 " corrugatus, Lam., India. 



Mr. Lea stated that in his first paper published in the Trans. Am. Phil. Soc. 

 in 1827, he paid attention to the difference of the teeth, and in 1829 he publish- 

 ed a description of that remarkable Unio from the Schuylkill, described under 

 the name of heterodon, from the very peculiar and aberrant form of the double 

 lateral tooth being placed in the right valve, and the single in the left one. 

 This was the first form of the kind which had ever come under his notice. A 

 few years subsequently he found the first specimen of an abnormal character, 

 conforming exactly to the normal condition of the heterodon, and this was in the 

 Bristol specimen. Since that period he observed closely the abnormal forms of 

 the species, and the result is given in the previous table. But it must be im- 

 pressed on the zoologist's mind, that the form of teeth which is normal in one 

 species, may be abnormal in another, because, while there is impressed on 

 every species a law as regards its form, and the general one of this species of 

 Unio is to have the lateral tooth double in the left, and single in the right 

 valve, and the cardinal either the same or double in both, yet aberrant forms 

 from this are quite numerous, as will be found in the following table of 

 species. He wished the attention of the members to the fact that what 

 was abnormal in some individuals of a species, would be perfectly normal 

 in others ; thus, in complanatus, when the lateral teeth are found double in the 

 right, and single in the left, they are reversed, and therefore abnormal ; but in 

 the heterodon this condition of the teeth is normal, and so it will be with other 

 conditions of other species, even so far removed from the typical Unio as in the 

 ezimius, Lea, from Siam, which has a treble lateral tooth in the left, and a dou- 

 ble one in the right valve as its normal form, for this is imitated by the speci- 

 men of corrugatus exhibited, which has the treble tooth in the left valve, and 

 double one in the right, which in this case is remarkable, its normal condition 

 being that of the typical Unio. 



In the following table will be found most of the species which, while they 

 are perfectly normal, are still aberrant from the typical Unio, all of them but 

 two having been described by Mr. Lea. 



Cardinal tooth single in both valves. 



Unio Bengalensis, Lea, Bengal. 



Cardinal tooth double in the right, and single in the left valve. 



Unio Corrianus, Lea, Bengal. 



" lamcllatus, Lea, Bengal. 



" bilineatus, Lea, Bengal. 



" contradens, Lea, Java? 



" gravidus, Lea, Siam. 



" tumidulus, Lea, Siam. 



" humilis, Lea, Siam. 



" Sagittarius, Lea, Siam. 



" substriatus, Lea, Siam. 



" Dunkerianus, Lea, Brazil. 



" Cambodianus, Lea, Siam. 



" eonsobrinus, Lea, China. 



" Layardii, Lea, Ceylon. 



" plicatulus, Lea, Borneo. 



" vittatus, Lea, Australia. 



" Wilsonii, Lea, Australia. 



" Mauriiianus, Lea, Indian Ocean. 



[Feb. 



