ZOOLOGY. 405 



species previously accounted distinct, is contributing towards its simplifica- 

 tion, and is, therefore, one of its truest benefactors. Having noticed some 

 of the most interesting physiological and zoological considerations which 

 connect themselves with the study of this group, the lecturer alluded, in the 

 last place, to its geological importance. Traces, more or less abundant, of 

 the existence of Foraminifera, ai - e to be found in calcareous rocks of nearly 

 all geological periods; but it is towards the end of the Secondary, and at the 

 beginning of the Tertiary period, that the development of this group seems 

 to have obtained its maximum. Although there can be no reasonable doubt 

 that the formation of chalk is partly due to the disintegration of corals and 

 larger shells, yet it cannot be questioned that in many localities a very large 

 proportion of its mass has been formed by the slow accumulation of forami- 

 niferous shells, sometimes preserved entire, sometimes fragmentary, and 

 sometimes almost entirely disintegrated. The most extraordinary manifes- 

 tation of this type of life, however, presents itself in the nummulitic lime- 

 stone, which may be traced from the region of the Pyrenees, through that 

 of the Alps and Apennines, into Asia Minor, and again through Northern 

 Africa and Egypt, into Arabia, Persia, and Northern India, and thence (it is 

 believed) through Thibet and China, to the Pacific, covering \cry extensive 

 areas, and attaining a thickness in some places of many thousand feet; an- 

 other extensive tract of this nummulitic limestone is found in the United 

 States. A similar formation, of less extent but of great importance, occurs 

 in the Paris basin; and it is not a little remarkable that the fine-grained and 

 easily-worked limestone, which affords such an excellent material for the 

 decorated buildings of the French metropolis, is entirely formed of an accu- 

 mulation of minute foraminiferous shells. Even in the nummulitic lime- 

 stone, the matrix in which the nummulites are imbedded, is itself composed 

 of minute Foraminifera, and of the comminuted fragments of larger ones. 

 The remarkable discovery has been recently made by Professor Ehrenberg, 

 that the green and ferruginous sands which present themselves in various 

 stratified deposits, from the Silurian to the Tertiary epoch, but which are 

 especially abundant in the Cretaceous period, are chiefly composed of casts 

 of the interior of minute shells of Foraminifera and Mollusca, the shells 

 themselves having entirely disappeared. The material of these casts, which 

 is chiefly silex, colored by silicate of iron, has not merely filled the chambers 

 and their communicating passages, but has also penetrated, even to its mi- 

 nutest ramifications, that system of interseptal canals, whose existence, first 

 discovered by Dr. C. in nummulites, has been detected also in many recent 

 Foraminifera, allied to these in general plan of structure. And it is a very 

 interesting 1 pendant to this discovery, that a like process has been shown, by 

 Professor Bailey, to be at present going on over various parts of the sea bot- 

 tom of the Gulf of Mexico and the Gulf Stream, casts of Foraminifera in 

 greensand being brought up in soundings with living specimens of the same 

 types. 



ON SOME UNUSUAL MODES OF GESTATION. 



The following is an abstract of a paper communicated by Dr. Jeffries 

 "Wyman to the Boston Society of Natural History (see Proc. Sept. 18-37), 

 on some unusual modes of gestation, which have been made by him the 

 subject of personal observation. 



Among Batrachians, the circumstances under which the young are devel- 

 oped, though less varied than in some of the other classes of vertebrates, 



