40 A, Agassiz—Connection between the Cretaceous 
than at others. The sensibly homogeneous character of such 
a tensile elastic film, would naturally tend to group the points 
of rupture, or jets of water, into more or less perfect order, 
during the process of cooling or of desiccation, Thus, the 
columnar structure of certain igneous rocks seems to be due to 
the tensile stress of contraction by cooling after solidification 
supervened ; while the analogous structure developed by the 
desiccation of homogeneous masses of moist clay, mud or 
starch, appears to be produced by a similar strain consequent 
upon shrinkage from loss of moisture. In a similar manner, 
the tense superficial capillary film of the water, when it expe- 
riences the sudden molecular impulse due to the emergence of 
elastic pulse, is ruptured along lines more or less symmetrically 
disposed on the surface of the water; and the liquid beneath 
is projected through these lines or points of least resistance. 
Berkeley, California, October 25th, 1881. 
Art. IV. — On the Connection between the Cretaceous and the 
recent Echinid Faune ; by ALEXANDER AGASSIZ.*  . 
preserved by many of the new genera discovered in 
water, and especially their resemblance to Cretaceous genera; 
and the study of the Challenger Echinids has brought this out 
still more plainly. 
e purpose of making the comparison of the Challenger 
Kchinids with the earlier Cretaceous types as complete as pos 
sible, it will be interesting to take a rapid review of the Cre- 
taceous Echinid fauna, and to contrast it with the abyssal 
fauna as a whole, independently of its combinations in time 
with the littoral and continental types, but not independently 
of its combinations with those types which extend into the 
eg fauna either from the littoral or from the continental 
auna. 
* From pages 25 to 30 of the Report on the Echinoidea of the Voyage of H. 
M. 8. Challenger, by A. AGASSIZ. 
