4 o VITREO US FORAM1NIFERA. PAKT in. 



margin of the shell runs into a keel, which is sometimes 

 extended into a knife-edge. Nearly all the Lagena 

 family are found in the North Atlantic and Mediterra- 

 nean, especially in the Adriatic, which is rich in species. 

 In the Nodosaria the cells which compose the shell 

 have so little connection one with another that they 

 may be easily detached ; which gives reason to believe 

 that the separation of the parts may be a means oi 

 reproduction and dispersion. 



The Globigerinidse are the most numerous family oi 

 the perforated series, and the most remarkable in the 

 history of the existing Foraminifera. They are distin- 

 guished by the coarseness of the perforations in theii 

 shells, and by the crescentic form of the aperture b^ 

 which the chambers communicate with each other. 



The genus Globigerina consists of a spiral aggrega- 

 tion of globose segments, which are nearly discon- 

 nected from each other although united by mutua] 

 cohesion. The segments are always somewhat flattened 

 against one another in their planes or junctions, and 

 sometimes the flattening extends over a pretty large sur- 

 face as in G, fig. 97. The entire series of segments shows 

 itself on the upper side, but on the lower side only the 

 segments forming the latest convolution are prominent 

 they are usually four in number, and are arrangec 

 symmetrically round a deep depression or vestibule ; the 

 bottom of which is formed by the segments of the earliei 

 convolutions. In this vestibule each segment opens b] 

 a large crescent- shaped orifice, the several chambers 

 having no direct communication with each other. Th( 

 entire shell of the ordinary type may attain the dia- 

 meter of about -^ of an inch, but it is usually mucl 

 smaller; the typical form, however, is subject to verj 

 considerable modifications. In newly formed segments 

 of Globigerina, the hyaline shell substance is perforatec 

 by tubuli varying from To * 00 to y^Vo f an i ncn " 



