CLASS II BRACHIOPODA 369 



The nature of the pedicle opening being employed for oi'dinal 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 (homoeomorphy) ; 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 

 of development, the austral group (Magellaniinae) passes through a different 

 series of loop metamorphoses from the boreal one (Dallininae).. 



It was by the application of the above-mentioned principles that Schuchert, 

 in 1893, arranged all the genera of Brachiopoda under the four orders 

 instituted by Beecher. Further attention has since been given to this subject 

 by the same writer, and the arrangement now oftered combines the brilliant 

 results obtained by Charles D. Walcott in his studies of the Cambrian forms 

 of all lands, and the important work of S. S. Buckman, relating to the 

 Brachiopods of Great Britain. 



Order 1. ATREMATA Beecher. 



This order includes primitive inarticulate, corneous or calcareophosphatic 

 Brachiopoda, with the pedicle usually emerging freely between the two valves. 

 Growth takes place mainly around the anterior and lateral margins. 

 Delthyrium or pedicle aperture originally unmodified, in later genera modified 

 by homoeodeltidia and pseudochilidia, or by thickened, striated and more or 

 less furrowed or even cleft vertical cardinal margins, the ventral cleft in most 

 specialised forms tending to enclose the pedicle and finally to restrict it to 

 the ventral valve ; when completely restricted the genera are referred to the 

 order Neotremata. Specialised forms tend to develop rudimentary articula- 

 tion or muscle platforms. The three other orders of Brachiopods have 

 arisen from the Atremata. 



Superfamily 1. RUSTELLACEA Walcott. 



Primitive, thich-shelled, corneous or calcareo-phosphatic Atremata, developing more 

 or less of homoeodeltidia and pseudochilidia. Muscle scars and vascular sinuses as a 

 rule not well defined in the shell. Out of this stock arose the Obolacea and Kutor- 

 ginacea. Cambrian and Ordovician. 



Family 1. Rustellidae Walcott. 



Primitive Eustellacea with the pedicle aperture of both valves small, more or less 

 open, and not much modified hy homoeodeltidia or pseudochilidia. Muscle scars and 

 vascular' sinuses not well defined in the shell. Lower Cambrian. 



VOL. I 2 B 



