PART III. MORPHOLOGY OF THE ORTHOID SHELL 



The Orthoidea, to be described in more detail later, are a prolific stock of primitive Protremata 

 which persisted, according to our present knowledge, from the middle part of the Lower Cambrian 

 to the end of Permian time. They are rather primitive brachiopods, certainly the simplest of the 

 "articulate" stocks, and are characterized by more or less strongly developed interareas in both 

 valves. Primitively the delthyrium was covered by a deltidium, but in later progressive stocks this 

 structure was lost and the delthyrium was unmodified except for lateral plates or sporadic apical 

 plates. 



This large suborder may be divided into two superfamilies on the basis of the shell structure, 

 whether fibrous and endopunctate or fibrous and impunctate. With these superfamily characters go 

 definite internal structures that are described in later pages. 



Most of the Orthoidea described in this volume were, at one time or another, classified under 

 the generic term Orthis. Even as late as 1892, the date of Hall and Clarke's great revision, not 

 more than thirteen orthoid genera were in use. These authors showed that Orthis (1828) had 

 become a "dump" genus, and that it embraced at least 13 groups of shells with orthoid features, 

 besides 10 other old or new genera with related forms. They further blazed the way toward a 

 correct genetic understanding of Paleozoic genera, a method that has been followed ever since and 

 one that has guided us in turn in the present revision. Schuchert (1897) regarded Hall and Clarke's 

 "groups of Orthis" as genera, and in the Bibliography of Schuchert and LeVene (1929) there are 

 catalogued as of July 1928 52 accepted genera of orthoids. In the present volume we recognize 

 103 genera or subgenera (29 are new) constituting the superfamilies Orthacea, Clitambonacea, and 

 Dalmanellacea. These statistics show also the accelerated rate at which students of brachiopods are 

 discerning the genetic ramifications of the orthoids. 



We believe that the present arrangement shows genetic relationships more clearly than here- 

 tofore and that our classification is more natural. But it can not be final as yet, and as years pass, 

 greater refinement in paleontologic and stratigraphic practice, along with the making of additional 

 collections from old fields and new, will enlarge our knowledge and tend to alter our scheme. We 

 hope, however, that our fundamentals are sound and that time will fill in either the details of the 

 scheme here presented or that of nature, since after all we are striving to learn nature's ways of 

 creation. 



MORPHOLOGY OF THE EXTERIOR 

 ORIENTATION OF THE SHELL 



In citing directions in or on a brachiopod shell, we shall use the terms dorsal and ventral, and 

 anterior and posterior (pedicle end). When discussing or figuring the posterior of a brachiopod, we 

 place the ventral valve down, as this is the correct biological orientation in the living animal. The 

 terms ventral and dorsal, therefore, indicate the vertical direction. In other words, these directions 

 are at right angles to the plane of the commissure, which is oriented by us in a horizontal direction. 

 The terms anterior and posterior, on the other hand, define directions parallel to the plane of the 

 commissure. These terms can therefore be applied with precision regardless of orientation, and thus 

 enable us to avoid the ambiguous terms front and back, down and up. 



In describing the shape, contour, or profile of a shell, the writers are following Thomson and 

 Buckman in giving preference to the dorsal valve. This leads to the use of an unfamiliar term for 

 some shells which are commonly called concavo-convex, as Dinorthis; according to our usage this is 

 a convexo-concave shell. No matter which valve be taken for reference, any precise nomenclature 

 must recognize a convexo-concave stage as well as a concavo-convex one. For the sake of uniformity, 

 then, we have adopted the scheme used by these two authorities on Mesozoic, Cenozoic, and Recent 

 brachiopods. 



