of the Amazons Valley. 105 



suitable for comparison with the more extreme forms of the 

 genus Anisothyris wliich occur fossil in this locality. 



We can, however, compare the most prevalent type of the 

 larger species (P. tenuis^ Gabb) with Azara laliata, D'Orb., 

 which it closefy resembles in general form. It differs, how- 

 ever, as already stated, in the direction of the umbones, and in 

 the recent shell being nearly equivalve, whereas the fossil 

 form is very inequivalve ; the cardinal tooth and socket are 

 likewise reversed ; furthermore the tooth in Azara is hollowed 

 out to receive a ligament, and is less strong and less curved 

 than in the fossil. The pallial border is entire, and the mus- 

 cular impressions agree with those of Azara. 



Anisothyris tenuis^ Gabb, oj). cit. pi. 10. fig. 1. 

 This is the most abundant form of the larger kinds of this 

 genus. To call it tenuis^ however, is as unnatural as to spell 

 odon with one o : the shell is essentially thick, both in sub- 

 stance and in ventricosity. It goes through several well- 

 marked varieties, however ; and, as our series comprises more 

 than one hundred specimens, I may perhaps be allowed to re- 

 christen it. In doing so I desire to perform an act of justice 

 towards Mr. Hauxwell, who, I am assured by Mr. Bates, is 

 a most deserving and indefatigable naturalist and collector, 

 and from whose labours much good has already resulted to 

 science and more may be anticipated. I propose to rename 

 this species 



Anisothyris Ilauxwelli, sp. nov. 

 PI. V. figs. 7 a, h, c, d. 

 Syn. Pachydon tenuis, Gabb & Conrad, op. cit. 



" Subtriangular, very oblique ; shell-substance thick in 

 adult specimens ; right valve profoundly ventricose ; umbonal 

 slope slightly angular, nearly terminal ; posterior extremity 

 truncated ; cardinal tooth broad and thick, erect, curved, with 

 an angular margin ; this tooth is overlapped in front by a 

 carinated and sulcated projection ; lunular depression pro- 

 found, very large, and broad." 



I propose to adopt the following varieties oi A. Hauxwelliy 

 represented by good series of well-marked forms, viz. 



Var. a. distorta. Var. /3. crassa. 



In variety a I have placed a large series of highly distorted 

 specimens which may OAve their singular form to having 

 sufiered by the periodic changes from extreme fresh water to 

 extreme salt water, due to their estuarine habitat. 



In variety /3 I place all the forms in which the extreme 

 Ann. & Mag. N. Hist. Ser.4. Vol.yii. 8 



