The test-ornament of Brachiopods is found in three main phases—smooth, ribbed, and spinous. These three phases are in this anagenetic sequence to one another:—in relation to its nearest allies, a costate species of a given series is more advanced than a smooth one of that series, and a spinose one still further than a costate. There are catagenetic developments also in reverse order:—in certain Productids the costate stage follows on a spinose: in Acanthothyris there are certain cases of the spinose ontogenetic stage being followed by a smooth. If, however, the catagenetic phases be put aside for the present, it may be said that the state of external ornament—smooth, costate, spinose—indicates the position of a brachiopod as more or less advanced than its fellows. The present paper is concerned only with two cases of the development of the ribbed stage. The passage of the smooth into the ribbed stage is not acquired in the same way : there are various methods by which similar looking ribbed forms have been evolved from similar looking smooth forms. This has a most important bearing on their classification. if the phylogenetie history of similar ribbed forms is demonstrably different, it is, according to modern opinion, a technical error to place them in the same genus, even if their interior organization be similar. if a given smooth form can have developed into one of the ribbed forms but not into the other, it is a technical error to unite it with the costate form into