During the past hundred years the structure and relation of Coccosteus decipiens AG. has been the subject of a careful study. Many papers were written and many reconstructions were made. To discuss all these works is not the subject of this paper, and therefore we shall only mention the two best known reconstructions - the one proposed by Traquair ( 1890), the other by Jaekel ( 1902). All the other reconstructions since that date are only more or less supplementary and modified redrawings of these two.
In spite of the fact, that they are based on the study of nearly the same material, they differ strongly in many ways (Fig.1 a and 1 b). Firstly is the shape of the head and body armours different as also their relative size. Secondly Jaekel has found some new plates in the head and body carapace. Thirdly the connection between the ventral and dorsal divisions of the body armour is drawn differently. Finally the position and form of the "pelvic girdle" and of other parts of the endoskeleton show many differences.
With the hope of finding which of these two reconstructions is the more correct I have examined a large number of Coccosteus in different museums. (The British Museum, The Royal Scotch Museum, The American Museum of Natura! History, The Buffalo Museum of Science, The Museum of Comparative Zoology in Harvard and The Paleontological Museum in Oslo). This investigation led me to believe that both recontructions are somewhat incorrect and that it is necessary to make a new one, combining the correct points of the older two. A great help to me was my study of the structure of Dinichthys - a form closely related to Coccosteus.
I shall below first discuss the headshield, then the structure of the body carapace and at last - the development of the endoskeleton.