THE NAUTILUS. 99 



elements of growth of this hiyer are diagonal to the general surface 

 of the shell. From the pallial line to the beaks is deposited the 

 fourth, or intra-pallial layer — the elements of which are parallel to 

 the general surface. The sectionized shell will show the extra- 

 pallial layer wedge-shaped, with the apex at beak, and base occupy- 

 ing the distance from the pallial line to the margin, while the intra- 

 pallial layer is also wedge-shaped, with its apex at the pallial line. 



Because the pallial line is composed of very many small muscle- 

 scars disposed in a line, if the two layers could be separated, a sur- 



face would be exposed " radially ridged." Sometimes, by decay, 

 this separation is effected, partially, near the beaks, and the " false 

 beaks " so exposed are strikingly " radially ridged " — so much so as 

 to deceive an expert like Dr. Lfa. If a thick-shelled Unio like 

 Quadrula trigona be burnt, this structure can be very readily demon- 

 strated. 



It is not impossible that this appearance of decayed or fossilized 

 Unios has given rise to the opinion, as stated by Mr. Chas. T. Simp- 

 son, that the primeval Unios were provided with " radial beak- 

 sculpturing." The difficulty experienced by every collector of ob- 

 taining living shells showing beak-sculpturing, and the a priori im- 

 probability of fossil shflls retaining this very perishable character, 

 lends an air of probability to the above theory, which may be further 

 strengthened by the curious fact that no North American Unio re- 

 tains the slightest tendency to show their beaks so sculptured. 



LAND SHELLS OF MX, DESERT, MAINE. 



BY II. S. COLTON. 



On Mt. Desert Island last summer I found land shells in six local- 

 ities. At Hall's Quarries I found Zonitoides arboreus near the shore 

 at the edge of the woods. From Seal Harbor I received Vitrea ham- 

 monis Strom, Pyramidula striatella Anth., Helicodiscus lineatus Say 

 and Carychium exiguum Say. At Coryledge point under boards 

 within a yard or two of the place where the beach began, I found 



