NATURAL SCIENCES OF PHILADELPHIA. 27 



Pebas. 



8 feet red and white clay and sand. 

 20 feet blue clay, 4 feet of fossils. 

 G inches coal. 

 15 blue clay. 3 feet of fossils. 



"I noticed that the lower strata had less univalves and more bi- 

 valves than the upper. I have sent you some shells that I know 

 are new, and bits of turtle shell, fish bones, coral, crustaceans, 

 etc., which have not been noticed in the bed before." 



I found no specimen of coral in this collection, nor in an}' other 

 of the Amazon fossils. 



Description of Shells of the Pebas Gy^oup. 



PACHYDON, Gabb. 

 In this genus the species are A'ery diverse in form, iuequivalve, 

 both beaks directed forward, spiral ; the cardinal tooth of the left 

 valve is covered in front throughout two-thirds of its length by a 

 portion of the exterior layer of the valves, which is transversely 

 striated, and resembles a wedge sunk into the base of the lunule, 

 so as to define the tooth from the exterior when the valves are 

 closed. In most species the prominent margin of the lesser 

 valve fits into a groove, the lower margin of which is a well- 

 defined lateral tooth. This character is entirely wanting in CoR- 

 BULA, as well as the external view of the cardinal tooth. In P. 

 cuneata this character is strongly marked. Mr. Gabb's name for 

 this genus must be retained, because Stuchbury's Pachyodon, as 

 well as Schumacker's, are superseded. 



P. (Anisorhyncus ?) dispar, Conrad, PI. I., fig. 1. 



Yery oblique, subquadrangular, right valve concave above, with 

 2 diverging carinated lines from apex, one directed towards the 

 ventral extremity, the other towards the anterior margin, left 

 valve profoundly ventricose, with a well-marked groove anterior 

 to the beak, directed somewhat obliquely backward to the ventral 

 margin which is emarginate. The two outlines represent side 

 views of right valve. 



Six or seven siaecimens of this singular species are before me, 

 and I supposed the valves belonged to two different species, so 

 ver}' remote in appearance the}' are from each other ; but one spe- 

 cimen, a cast, occurs with both valves represented in connection. 



