The Ontogeny and the Evolution of Larger Foraminifera

The phylogenetic development of each form of the foraminiferal families and subfamilies dealt with in this article is well recapitulated in its ontogenetic development. The most primitive form of each family evolved from a certain simply constructed form of another family by the addition of a new structural constituent, such as the median chambers, the lateral chambers, the secondary septa, etc. Similarly, every form which is regarded to be more advanced than the others in every family arose from the next succeeding less advanced form by the addition of a new structural constitutent, such as in the last-cited case or the multiplication in the number of a certain structural element, such as the partition of the nucleoconch and the septa. The accrued structural constituent is never divided into two stages toward the periphery of the test. Quadrangular chambers are never succeeded by hexagonal or spatulate median chambers. The spiral structure appears to be less advanced than the annular structure. Because the former is always succeeded by the latter in all the families. The bilateral-symmetrically disposed nepionic chambers of some families, such as Lepidocyclinidae, Lepidorbitoididae and Miogypsinidae, were formed when the last-formed chamber of the first nepionic coil made a remarkable development and it served as a principal auxiliary chamber like the second nepionic chamber which made the other principal auxiliary chambers. The bilateral-symmetrical structure just cited is considered to be more advanced than the spiral structure and the former arose from the latter. The ontogenetic acceleration of the number of the nepionic spires and the nepionic chambers is well traced in the genus Miogypsinoides. The Permian Neoschwagerininae became extinct when it attained the culmination of the structural complicacy, whereas the genus Nummulites which ranges from the Paleocene to the Rupelian attained its evolutional climax in the structural specialization and in the volumetric augmentation in the Lutetian and not in the Repelian