368 MOLLUSCOIDEA ■ phylum v 



articulating processes. These divisions, " Articules and Libres," were recog- 

 nised by Deshayes as early as 1835, l)ut not until twenty-three years later 

 were the names Lyopomata and Arthropomaia given them by Owen. These 

 terms have been generally adopted by writers, though some prefer Inarticulata 

 and Articalafa Huxley, or Bronn's Ecardines and Testicardines. Bronn (1862) 

 and King (1873), while retaining these divisions, considered the presence or 

 absence of an anal opening more important than articulating processes, and 

 accordingly proposed the terms Pleuropygia and Apygia, and Tretenterata and 

 Clistenterata respectively. In many Paleozoic rostrate genera of Clistenterata, 

 it has been shown that an anal opening was also present, and therefore the 

 absence or presence of this organ is not of ordinal value. 



The first attempt to construct a classification of the Brachiopods was that 

 of Leopold von Buch, who took for his principal differential characters the 

 conformation of the umbonal region, the presence or absence of a pedicle, the 

 nature of the deltidium, and the external form and ornamentation of the shell. 

 While his classification does not reflect a perfect understanding of the features 

 in question, it is remai'kable that von Buch, nearly eighty years ago, and 

 Deslongchamps, twenty-eight years later, recognised some of the principles 

 upon which the classification of the Brachiopoda is now established, as, for 

 example, the nature of the pedicle opening. 



Up to 1846 the general external characters of the Brachiopods served 

 the majority of authors as the essential basis for generic differentiation. In 

 that year, however. King pointed out that more fundamental and constant 

 characters exist in the interior of the shell, a fact which soon came to be 

 generally recognised, mainly through the voluminous and admirable contribu- 

 tions of Thomas Davidson. 



AVaagen in 1883 found it "absolutely necessary" to divide Owen's two 

 orders into seven suborders. The basis for these suborders rests on no 

 underlying principle of general application, and yet five of these divisions are 

 of permanent value, for each contains an assemblage of characters not common 

 to the others. 



No classification can be natural and permanent unless based on the history 

 of the class (chrono genesis) and the ontogeny of the individual. However, as 

 long as the structure of the early Paleozoic genera remained practically un- 

 known, and the ontogeny wholly uurevealed, nothing of a permanent nature 

 could be attempted. In the excellent volume by Hall and Clarke (Palaeontology 

 of New York, vol. viii., 1892-95), the great majority of the Paleozoic genera 

 are clearly defined. The ontogenetic study of the Paleozoic species was 

 initiated in 1891 by Beecher and Clarke, followed by Beecher, and more 

 recently by Schuchert ; and their results combined with those derived from 

 the study of the development of some living species, such as have been 

 published by Kovalevski, Morse, Shipley, Brooks, OEhlert, Beecher and 

 others, confirm the conclusion reached through chronogenesis. Moreover, the 

 application by Beecher of the law of morphogenesis, as defined by Hyatt, 

 and the recognition and establishment of certain primary characters have 

 resulted in the discovery of a fundamental structure of general application for 

 the classification of these organisms. It has for its foundation the nature of 

 the pedicle opening and the stages of shell development. On this basis 

 Beecher (1891) has divided the class into four orders: the Atremata, Neo- 

 tremata, Protremata and 2'elotremata. 



