SECT. ii. VITREOUS FORAM1NIFERA. 39 



apart, they are always connected by intermediate forms : 

 but there is no such connection between the two great 

 natural orders, which are not only separated by the 

 tubuli in the shell, but in many instances by the struc- 

 ture of the interior and the corresponding character of 

 the animal. 



In the Lagenidse, which form the first family of the 

 Perforated order, the vitreous shell possesses great 

 hardness, and is pierced by numerous small tubuli. It 

 is very thin, and of glossy transparency. The first four 

 shells in fig. 97 represent some of its forms. 



The genus Nodosaria has a very extensive range of 

 forms, from the elongated structure to the nautiloid 

 spiral, depending upon the relative proportions and 

 arrangement of the segments. The segments are se- 

 parated by constrictions transverse to the axis of growth > 

 or by bands as in the Nodosaria rugosa, B, fig. 97. It 

 frequently happens that parts of the shell are not per- 

 forated ; and there are generally longitudinal ribs which 

 sometimes have spines projecting from every part of the 

 interior, as in Nodosaria spinicosta, c, fig. 97. 



In the genus Nodosaria, the axis of growth changes 

 from a straight line to that of a spiral, so that the 

 septa or divisions between the segments cross the axis 

 obliquely, and the aperture instead of being exactly 

 central becomes excentric. Between these extremes 

 there is a numerous series of gradations. The Cristal- 

 laria is the highest type ; the form is a nautiloid spiral, 

 more or less compressed (D, fig. 97), of which each whorl 

 has its chambers extended by winged projections so as 

 to reach the centre, and entirely encloses the preceding 

 whorl. The number of chambers in each whorl is 

 much smaller than in most of the nautiloid spirals, not 

 being more than eight or nine. The divisions are 

 always strongly marked externally by septal bands, 

 varying in character according to the species. The 



