•454 PKOCEEDIKGS OF THE GEOLOGICAL SOCIETY. [April 18, 



various graduating forms with Marginidina Sublituus (fig. 37)an d 

 Planidaria Bronni (fig. 40), ou the one hand, and with Planidaria 

 reticulata (fig. 38) and PI. pauperata (fig. 39) on the other. 



PI. pauperata is here indicated as a variety not previously recog- 

 nized. It is a minute, subtriangular, flattened shell, consisting of 

 six chambers, four of which are transversely broad ; the two others 

 (the oldest) are subglobular, and resemble the early cells of a Nodo- 

 saria. The shell is smooth, the septa are gently curved and sulcate ; 

 the aperture is distinctly marginal and produced. This is one of 

 the simplest forms of the Cristellarian type. Although very minute 

 (about -g-jy-^ in. in diam.), it closely assimilates in outline to the large 

 Planularian Oristellario} of the Subapennine tertiaries, which some- 

 times attain to y 1 ^ in. in diam. 



At first sight it might appear difficult to suppose that all the 

 varieties, ranging from the simple Nodosaria to the nautiloid Cristel- 

 laria (figs. 1-43), should belong to only one species ; but such an 

 exceedingly wide range of variation is the rule in Foraminifera — a 

 group of creatures extremely low in organization, and showing a 

 licence as to their morphological laws, as great as any possessed by 

 the lowest animals and plants. 



Similar in shell-structure, the Nodosarince differ amongst them- 

 selves in their mode of growth chiefly as to the rectilinear or spiral 

 arrangement of the chambers, as to the more or less excentric posi- 

 tion of the aperture, and as to the flattening of the shell. In every 

 gathering of Foraminifera rich in this group, the gradations of form 

 are endless, presenting, however, certain more or less conspicuous 

 varieties, which are conveniently separated as subspeeific or varietal 

 groups, and which have been considered by some to be of generic 

 and specific value. Moreover each local fauna has its own set of 

 varieties, which are often sufficient to serve as characteristic features 

 of the fauna. 



With the exception of Flahellina rugosa, Vaginulina strigillata, 

 Planidaria reticulata, PI. pauperata, and PI. Bronni, all the above- 

 named varieties of Nodosarina occur more or less abundantly in the 

 recent seas, as well as in tertiary and secondary deposits : nor are 

 the excepted varieties without their representatives ; for they do not 

 differ in any essential character from many recent forms with which 

 we are acquainted. 



Polymorphina. — The well-known species Polymorphina lactea is 

 represented in the clay from Chellaston by a very minute but charac- 

 teristic individual (PI. XX. fig. 44). It is a short tear-shaped shell, 

 being of the variety known as Outtulina communis (D'Orb.). Poly- 

 morphina occurs also in the Lias, where it is represented by Mr 

 Strickland's rather more elongate variety, P. liassica* ; and it is con- 

 tinued to the present day in great varietal abundance. 



Bidimina. — A cast of a minute Bulimina (fig. 45) resembling B. 

 Pyrula of D'Orb. also occurs. It is not unusual to find the very 

 thin-shelled small individuals of this species indicated by casts only 



* Quart. Journ. Greol. Soc. vol. ii. p. 30. 



