CHAPTER V. 



OF THE FAMILY MILIOLIDA. 



80. The series of generic types which is marked out by the porcellanous texture of the 

 shell {% 57), and by those other structural characters which are associated with that dis- 

 tinctive feature {%% 62 — 65), includes so great a variety both of modes of conformation and 

 of grades of development, that at first sight the association of all these into a single family 

 would seem altogether unnatural. As we proceed in our study of them, however, we shall 

 find that from the lowest to the highest of these forms each is most remarkably connected 

 with other parts of the series by links of affinity so strong as to forbid their dissociation ; so 

 that, starting from the humblest or simplest types, we are gradually conducted, with scarcely 

 any decided interruption, to the highest or most specialised. Thus in SquamuUna (Plate I, fig. 

 22) we have a raonothalamous shell, of which the shape, although not very definite, seems to 

 preclude any extension or super-addition. In Cornuspira (Plate V, fig. 1 6) we find that the shell, 

 although still remaining monothalamous or undivided, is indefinite in its mode of increase, 

 receiving a succession of increments which foreshadows the successional addition of new 

 chambers in the Polythalamia. From this undivided spiral to the regular, scarcely divided 

 spiral of certain " spiroloculine " forms of Mlliola, the transition is almost insensible ; from the 

 " spiroloculine" we pass by eas)' steps to all the other forms of the Milioline type; and certain of 

 the most aberrant of these establish the transition to Hauerina and Fahidaria, in which last the 

 Milioline type seems to reach its most complex phase, the cavity of the shell being minutely 

 subdivided, as in Orbiculina and Alveolina, and the single large orifice into being replaced 

 by multiple pores. Returning to our starting point, we shall find in the proteiform Nubecu- 

 Jaria (which seems as much related to SquamuUna as to Cornusjnra) a sort of primitive sketching- 

 out of the various plans of growth which are more perfectly evolved in higher parts of the 

 series ; all its forms, however, being obviously but varieties of one type, of which the most 

 definite positive character is afforded by the incompleteness of the separation between the 

 successive chambers. The substitution of the rectilineal for the spiralis more definitely mani- 

 fested (though still under a great variety of aspects) in Vertcbralina, and the division of the 

 chambers is more complete. From this we pass almost insensibly to Penerojjlis, which also 

 presents itself under the form of a spiral giving place to a straight line, and in which the 

 elongated fissure that constitutes the aperture in Vertcbralina is broken up into a row of 

 separate pores. The subdivision of the chambers of Peneroplis by secondary partitions con- 



