As Paterina is the most primitive form of all, we may place it at the root of the phylogenetic tree. From it sprang the Atremata, which gave off the Neotremata and Protremata; the most primitive Neotremata seem to be the Trematidae, while the connecting link between the Protremata and Atremata is furnished by the Kutorginidae. From the genus Conchidium and its allies we may see how the Rhynchonellidae ushered in the Telotremata as an offshoot from the Protremata. The Telotremata subsequently gave off two main branches, which became specialised with the loop-bearing and spire-bearing forms respectively.

The evolution and mutual relationships of genera have been indicated with much probability by Hall, Clarke, and others. The Obolelloid type may be connected with the Linguloid by means of Lingulella and Linyulepis, while in Lingula itself we find the point of divergence for the ancestors of Trimerella, and for a line of variation culminating in Dignomia. The Palaeozoic Rhynchonelloids branched off at an early period from the same stock as Orthis, and are connecting links between this genus and Mesozoic Rhynchonellae; and a whole series of genera exhibit intermediate stages of structure between the Rhynchonelloid and Pentameroid groups. The Terebratuloids can be traced back to the primitive type Renssoellaria; and amongst spire-bearing forms, the protean genus Spirifera can be split up into groups of species which diverge along lines tending to forms no longer congeneric. When we come to deal with specific differences we find frequently such a host of intermediate varieties that the separation of many species, as in the case of Mesozoic Terebratulae, is to a large extent arbitrary and artificial.

INDEX

References to figures are printed in thick type (248, 197); to systematic position, in italics (391, 430)