1869.] 289 [Shaler. 



Length of carapax in a male, excluding the teeth of the front, 9.3 

 rnillim.; breadth, 11.2 millim.; ratio of length to breadth, 1:1.20; 

 length, including spines of the front, 10.0 millim. Another male 

 gives, length of carapax, 7.3 millim; breadth, 8.5 millim.; ratio, 

 1: 1.16; length including spines, 7.8; length of meros in larger cheli- 

 ped, 6.4; length of hand, 10.4; breadth of hand, 4.3; length of mores 

 in smaller cheliped, 5.8; length of hand, 9.4; breadth of hand, 3.3. 

 Length of carapax in a female, 8.0 millim. ; breadth, 9.6 millim.; ra- 

 tio, 1:1.20; length including spines, 8.6 millim. 



In young specimens the sides of the carapax are less convex than 

 in adults. 



Pacheca, one of the Pearl Islands, Bay of Panama, six to eight 

 fathoms among pearl oysters; F. H. Bradley. 



Q. coronata Dana, the only other described species of the genus, 

 was from the Sooloo Sea, or Balabac Straits. 



Notes on the Concextric Structure of Granitic Rocks. 

 By N. S. Shaler. 



Von Buch, in his paper on the concentric form of massive rocks, 

 called attention to the peculiar onion-like lamellation visible upon the 

 outside of almost all granite masses; he left, however, the question 

 of the origin of this peculiar structure quite unexplained. It proba- 

 bly afforded him another proof of the theory of upheaval of volcanic 

 and other dome-like elevations, an opinion which it oftentimes seems 

 to support. So far as is known to the author, though several wi-iters 

 have referred to the existence of this pecuUar feature in certain 

 massive rocks, no effort has been made to trace the cause of its 

 existence. 



As affecting the disintegration of granitic and other massive rocks 

 where it occurs, this feature in their structure has a very great im- 

 portance. Being most marked in those regions where the ordinary 

 joints and Assuring have had the least effect in preparing the way for 

 detrital agents, it enables flowing water, frost and waves, to break 

 open masses which, but for this element of weakness, would be very 

 little affected by the agents of decay. The shore of New England 

 and the more northern portions of the Continent of North America, 

 abound with cliffs of the hummocky form so characteristic of granitic 

 shores, which, on inspection, show how far this feature renders the 

 pounding of the sea effective. Looking closely at the structure of 

 any portion of such a shore line, Ave perceive that the waves, finding 



PROCEEDINGS B. S. N. H.— VOL. XII. 19 APKII., 1869. 



