304 MOLLUSCOIDEA SUB-KINGDOM v 



While his classification lacks a perfect understanding of the features in question, 

 it is remarkable that von Buch, nearly sixty years ago, and Deslongchamps, 

 twenty-eight years later, recognised some of the* principles upon which the 

 classification of the Brachiopoda is now established, viz. 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. 



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

 orders into seven sub -orders. The basis for these sub -orders 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 (chronogenesis) and the ontogeny of the individual. However, as 

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

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

 could be attempted. In the recent and very excellent volumes by Hall and 

 Clarke (Palaeontology of New York, vol. viii.), the great majority of the 

 Palaeozoic genera are clearly defined. The ontogenetic study of the Palaeozoic 

 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 growth. On this basis Beecher 

 (1891) has divided the class into four orders: the Atremata, Neotremata, Pro- 

 tremata, and Telotremata. 



The nature of the pedicle opening being employed for ordinal divisions, 

 persistent internal characters of the shell are, as a rule, used for superfamily 

 purposes. Such are the presence or absence of a spondylium, brachial supports, 

 etc. Family divisions are based upon a combination of external and internal 

 generic characters, such as the outer form, nature, and position of muscles, 

 internal plates, etc. 



No division, however, has any value unless tHe group contains forms of 

 but one phylum, since a phylum or line of descent cannot originate twice. 

 However, it happens that the same or nearly the same combination of mature 

 characters is developed along different lines ; and when this occurs the 

 ontogeny will show it. It is therefore not correct to group different stocks 

 under one and the same genus. For instance, the family Terebratellidae 

 probably divided during early Mesozoic times, one stock drifting into boreal 

 and another into austral regions. These two stocks agree structurally in the 

 earliest shelled condition and also at maturity ; but between these two stages 



