104 Mr. H, Woodward on the Tertiary Shells 



tary of the Royal Geographical Society, whose researches on 

 the Amazons have added such vast stores, both of knowledge 

 and specimens, to aid the natural-historian. 



Shell highly nacreous, with a veiy thin external cellular 

 layer, tumid ; umbones minute, compressed, scarcely percep- 

 tible above the hinge-line ; hinge-line straight, nearly two- 

 thirds the entire length of the shell (hinge-line 1| inch long 

 in the specimen figured) ; posterior border one third deeper 

 than the anterior. Greatest length 2f inches, greatest depth 

 If inch; depth of valve at narrowest side If inch, thickness 

 1 inch. 



Mr. Conrad speaks of " fragments of a singular bivalve, 

 probably allied to Miilleria, one of which is pearly as a Unioj 

 and has a naiTow elongated muscular impression, veiy different 

 in size and outline from that of Millleria.'''' Possibly Mr. 

 Conrad may have seen fragments of this Anodon. 



We have compared Anodon Batesii with A. politus from 

 Siam, and A. Kellettii^ and with a new species from Bareilly, 

 India. All these, however, are thicker in the umbones and 

 much shorter along the hinge. The South- American Anodons 

 are all very different in form from A. Batesii^ so far as the 

 means of comparison in the British ]\Iuseum enable us to 

 judge. 



Fam. Corbulidae. 

 Genus Pachydon, Gabb. 



" The name Pachydon^'' writes Mr. Conrad, " is objection- 

 able, in consequence of its derivation being the same as 

 Pachyodon ; and I have been requested to substitute another." 

 He adds, " If naturalists object to Mr. Gabb's name, I would 

 suggest Anisothyris (unequal valves) to take its place." The 

 objection to Pachydon is too obvious to need any further 

 delay in condemning it : we therefore beg leave to adopt for 

 the genus the name Anisothyris^ Conrad, in its place. 



" The hinge of this genus is very similar to that of Corhula, 

 much more so than to that of Azara ; but the spiral beaks are 

 in marked contrast to those of Corhula^ 



In comparing the shells of Anisothyris [Pachydon) with 

 Corhula, Azara, Necera, and Cardilia, &c., it is curious to 

 observe that the recent species all have the umbones directed 

 towards the posterior (siphonal) end ; in the fossil species, on 

 the contrary, the umbones look towards the anterior side. We 

 find also that the cardinal tooth is in the left valve in the 

 recent, and in the right valve in the fossil shell, and the 

 socket vice versa. It is difficult to suggest any recent shells 



