4 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



(4) We used serial sectioning as a last resort, since this method usually means the total loss of 

 the specimen. We developed a method, however, in the study of the pentameroids, by which we 

 were able to save a thin section of the shell as a result of serial sectioning. This method is described 

 in the chapter on the Pentameroidea, as we were not obliged to use it to any great extent in the study 

 of the Orthoidea; it is of great aid among the rhynchonellids, spiriferids, and terebratulids, where 

 the interiors are of hard cemented rock. 



Species Lists.— Throughout our paper we have listed under each genus the known species 

 which we believe have the structure of the genus as exhibited by the genotype. In some instances 

 our lists are complete, in others they are quite incomplete. Among the Cambrian brachiopods, for 

 example, there are a goodly number of species left unplaced because there was no hint of the internal 

 structure in the specimens available to us, and we would not place any of the species unless we could 

 feel reasonably sure that our reference was correct. Our lists are incomplete also because we made 

 no systematic attempt to track down in the literature all of the species of the Orthoidea and Pentam- 

 eroidea. Although this might have been desirable, we would doubtless have had to leave a large 

 number of these unplaced, too, because of lack of material or inadequate descriptions and figures, 

 particularly in the older literature. Therefore we placed only the species that came under our direct 

 observation. Some familiar species we left out because we could not establish any substantial generic 

 reference for them. An example is "Orthis" eminens, which has been placed by various writers in 

 Rhipdomella or Dalmanella, but belongs to neither. It appears to be closest to Idtorthis in internal 

 structure but varies too notably to be placed there. It appeared to us better to admit honestly that 

 we did not know what to do in certain instances. After us there will be many readjustments and many 

 new genera will ultimately gather home the flocks we have left to wander. 



Genetic Lines. — A note regarding the diagrams of generic evolution which appear in the dis- 

 cussion of the families is necessary. These diagrams are designed to be suggestive and not final. 

 There is probably only one family described by us whose evolution has been fairly well established, 

 and that is the Schizophoriidas. But there are difficulties even here: we do not know the Middle 

 Ordovician (Chazy) progenitor of the group, nor do we know definitely the origin of Enteletes, 

 whether directly from Schizophoria or indirectly from that genus through Orthotichia. Our family 

 Plectorthids can be traced to Finkelnburgia and possibly to Orusia, but the antecedents beyond 

 those genera we can not find. It must be borne in mind that the late Cambrian and early Ordovician 

 brachiopods are virtually unknown and that here occur the genera that bridge the genetic gaps 

 between the Middle Ordovician (Chazy) and the Upper Cambrian. It might be suggested, too, 

 that the Chazy brachiopods are none too well known. Accordingly we find ourselves without sure 

 footings among the primitive brachiopods upon which to build our evolutional structure. It is there- 

 fore unwise at this time to link the known Cambrian forms with those of later time. 



We might again cite an example. It has been maintained by some writers that the genus Billings- 

 ella gave rise to the Strophomenacea, and Eostro-phomena, a Lower Ordovician shell, has been 

 indicated as the forerunner of that great division. We now see that Eostrofhomena is an orthid. 

 Furthermore, genuine Strophomenacea are not known till Middle Ordovician (Chazy) time. 



At most, what we are trying to do is to show that the indicated evolution is structurally possible, 

 but we can not say that our scheme represents the actual course of evolution. Our genera are, there- 

 fore, rather iceloplasmic types^ having the structure capable of producing the evolution indicated, but 

 this does not necessarily define the actual course of brachiopod genesis. 



Acknowledgments. — During the progress of our work it has often been necessary to call on 

 specialists and others in order to see more material and to obtain advice along various lines. _ Every- 

 where we turned for assistance we have been aided very cheerfully. Miss Helen M. Muir-Wood 

 of the British Museum kindly loaned us an excellent series of specimens of the genus Schizophorella; 

 Doctor A. H. Westergaard of the Geological Survey of Sweden, at Stockholm, sent us for study 

 specimens of "Dalmanella" testudinaria which enabled us to make the astonishing discovery that the 

 chosen type of Dalmanella was different generically from all other American species referred to this 

 genus except one. Doctor Westergaard also loaned us an exquisite series of Orthis punctata showing 



^ Kirk, Amer. Jour. Sci. (5), vol. 18, 1929, p. 345. 



