CHAPTER YII. 



OF THE SUB-ORDER PERFORATA. 



225. A very large proportion of these types of Foraminifera which present themselves 

 to general observation, — including nearly the whole of those inhabiting our own shores, with 

 the exception of the 2JilioUe, — belong to that division of the Foraminifera which is charac- 

 terised by the possession of a shell perforated by tubular openings for the exit of pseudopodia. 

 The reasons already specified (Chapter III) appear to justify us in ranking this division as a 

 Sub-Order, equivalent in value to the "imperforate" division (^ 75) ; but whilst the subdivision 

 of the latter into families may be very naturally based on the differences in the material of 

 the shell, no such characters are available in the present case, the shell being everywhere 

 calcareous, and being composed of vitreous or hjaline substance, except in a small group in 

 which it is occasionally, but not essentially, arenaceous. Notwithstanding the very close resem- 

 blance in external form and in general plan of growth (sometinjes amounting to absolute 

 ixoiiwrpliisw), which often presents itself between the members of the " porcellanous" and of 

 the '•' vitreous" series {% 67), there is no real gradation between them. Such a gradation does 

 exist, however, between the " arenaceous" forms of the Perforata and the Imperforata 

 respectively ; for whilst, as we have just seen, it may be doubted whether Vnkmlina, in spite 

 of the essentially arenaceous composition and imperforation of the external test in its fully 

 developed condition, should not rank among the former in virtue of the perforated hyaline 

 layer of shell which immediately covers its sarcode-body, there seems almost equal reason 

 for ranking among the latter some of those TexhdaricB, in which the pores, large and distinct 

 as they arc in the original shell, seem entirely )ilocked-up by arenaceous incrustation. 



226. But although we do not find in this Sub-Order any such well-marked differences 

 in the material of the shelly envelope, as those which serve to characterise the families of the 

 Imperforate series, yet it does not seem very difficult to group its diversified forms around 

 certain principal types, so as to assemble them into Families, of which the members are recog- 

 nisable by their mutual relationship, and by their essential dissimilarity (except in the border 

 forms) to those of other groups of equal rank. It is not possible, in the present state of our 

 knowledge, to separate these families by precise definition ; for they arc so connected with 

 each other by, intermediate forms, that any definition which should be founded upon the 



