Crosby.] 7 160 [March 5, 
Saussure’s measurements are : length of carapax, 1 in. 7 lines; 
breadth, 2 in. ; ratio, 1: 1.26. 
Stimpson’s measurements are: length of carapax, 1.15 in.; breadth, 
1-46 in.; ratio, 11:27. 
Other than this variation in proportions, which may be due to age, 
the specimen agrees well with the descriptions and figures quoted 
above. The following additional characters may however be noted: 
External maxillipeds hirsute, the ischium toothed on the inner mar- 
gin; chelipeds externally tuberculate, the tubercles growing smaller 
and exhibiting a tendency to arrange themselves in rows below; a 
strong spine arises from the inferior margin of the propodus near the 
base and extends horizontally outward and forward. The dactylus of 
the (in our specimen) right hand is compressed, with a crest of 
rounded teeth above, the first being larger than, and somewhat dis- 
tant from, the rest; a strong curved tooth projects downward from the 
outer surface of the dactylus near the articulation with the propodus. 
The second and third joints of the abdomen are broader than the 
following ones, the second being tuberculate; the last joint is a third 
longer than broad. 
On A POSSIBLE ORIGIN OF PETROSILICIOUS ROCKS. 
By W. O. Crossy. 
g, and, geologically at least, one of the 
most important, results of the deep sea investigations carried on dur- 
ing the last decade, particularly by the Challenger Expedition, is 
the discovery that over very extensive areas in the deeper parts of 
the ocean, two remarkably fine and uniform and yet very dissimilar 
kinds of sediment are slowly accumulating. Of these, the first dis- 
covered and the best known is the globigerina-ooze, which appears in 
the dredge as an impalpable and tenacious calcareous slime or mud, 
consisting essentially of the shells of Globigerina and other Forami- 
nifera, though often perceptibly silicious from the presence of 
sponge spicules, radiolarian skeletons, and diatom frustules, and 
frequently containing the hard parts of higher animals. This 
material forms a white powder on drying, and it is now generally 
recognized as a modern chalk deposit; a slight degree of induration, 
and a segregation of the silica being all that is required to give it the 
aspect of the true chalk with flints; and in many parts of the ocean 
its deposition has probably been uninterrupted since Cretaceous time. 
One of the most interesting 
