P. 521), palaeontologists themselves have in this branch not very closely followed the progress of their own science, since Dr Ruedemann, in regard to his new Pollicipes siluricus, 1901, speaks of " the enormous gap existing between the appearance of this Lower Siluric type and the next Upper Triassic (Rhaetic) representatives of the genera Pollicipes and Scalpellum."
A more generally accepted view - especially among palaeontologists - is the tritubercular theory, according to which the most generalized type of tooth consists of three cusps arranged in a triangle, with the apex pointing inwards in the teeth of the upper jaw.
Possibly mammalian remains also occur in the antecedent Triassic epoch, some palaeontologists regarding the South African Tritylodon as a mammal, while others consider that it was probably a reptile.
It is the general practice of palaeontologists to regard each graptolite polypary (rhabdosome) developed from a single sicula as an individual of the highest order.
Vertebrate palaeontologists were slow to grasp this principle; while the early speculative phylogenies of the horse of Huxley and Marsh, for example, were mostly displayed monophyletically, or in single lines of descent, it is now recognized that the horses which were placed by Marsh in a single series are really to be ranged in a great number of contemporaneous but separate series, each but partially known, and that the direct phylum which leads to the modern horse has become a matter of far more difficult search.