The contribution of paleontology to the study of evolution must come primarily through exposition of the actual course of organic development as revealed by fossil remains. Groups vary widely in the excellence of their fossil record, however, and it is evident that relatively few among the host of plant and animal lineages are well suited for phylogenetic study. Fortunately some groups, especially among marine invertebrates, have left records of superior quality. From among these the brachiopod order Terebratuloidea has been selected as the subject of this study. The shells of terebratuloids (and apparently all other articulate brachiopods) consist exclusively of the relatively stable mineral, calcite, and thus tend to be well preserved. The existence of living representatives of the group has made possible investigations of the anatomy and of the relationship between soft and hard parts. It is known therefore that there is an intimate relationship between certain vital soft parts and some of the skeletal features. Further the group has an extremely long known history for it ranges from the higher Silurian to the present.2 Several of the skeletal features of Terebratuloids are known to undergo progressive change. The modifications of the loop are of the greatest interest because they reflect the nature of the lophophore and it is the purpose of this article to outline the historical development of the terebratuloid looD insofar as it is now known.