228 



ing the influence that the geology of a country exerts on the 

 distribution of its inhabitants. A careful study of the geological 

 character of a new country would teach lis, before a single hut 

 had been erected, where the principal towns and cities would be 

 built. Very sincerely, yours, 



" Francis Mason." 

 " Newton Centre, Aug. 7, 1855." 



The following descriptions of shells, by Dr. A. A. Gould, 

 should have accompanied those on p. 127, but the manu- 

 script was accidentally overlooked : — 



Unio bracteatus. T. transversa, oblongo-ovata, solidula, 

 sub-ventricosa, tenuiter striata, flavo-viridis, viridi saturation 

 radiata, antice rotundata, postice subacuta ; umbonibus ad trien- 

 tem anteriorem sitis, elevatis, utroque angulatis ; marginibus 

 dorsalibus et ventralibus lente arcuatis ; dentibus cardinalibus 

 perobliquis, compressis, duplicibus ; dentibus lateralibus arcuatis ; 

 margarita argentea, aureo bracteata ; limbo iridescente. Long. 

 2i; lat. 1 ; alt. 1| poll. From Llanos River, Upper Texas. 

 Dr. T. H. Webb. 



In size, shape, and color, like V. Ogeecheensis, but the nacre 

 and position of the beaks are different. U. Hydianus is more 

 inflated, more inequilateral, nacre pearly white. U. Menkeanus is 

 more compressed, tip more acute, tooth in left valve deep cleft. 



Unio petrinus. T. solida, ovato-quadrata, subequilateralis, 

 antice acute rotundata, postice late truncata, subtus vix arcuata, 

 prope marginem dorsalem posticum plicis obliquis minimis 

 ornata ; umbonibus modicis, subplanulatis ; epidermide dilute 

 flavo-viridi ; dentibus cardinalibus robustis, pyramidatis, laterali- 

 bus curtis ; margarita alba. Long. 1^; alt. 1^-; lat. f poll. 

 Llanos River, Texas. Dr. T. H. Webb. 



May be compared to U. Lamar ckianus. Lea, (=z U. Aherti, 

 Conr.) It is less compressed, apices less acute, and less angu- 

 lar ; the posterior altitude is greater, the posterior dorsal margin 

 being less declivous, but as the posterior truncation is less 

 decided, the length is greater compared with the height ; the 

 surface is irregular, but destitute of granules or nodules. 



