RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS $2 J 



Of these, the first two orders belong to the Upper Silurian, while the 

 third is Devonian. 



The Dorsal Head-Shield of the Osteostraci. 



Of the three orders above-named, the Heterostraci and Osteostraci 

 are the oldest, and among them the Cephalaspidee have afforded the 

 most numerous and best worked-out specimens. At Eootzikiill, in 

 the island of CKsel, the form known as Thyestes (Amhenaspis) verru- 

 cosus is especially plentiful, being found thickly present in among the 

 masses of Eurypterid remains, which give the name to the deposit. 

 Of late years this species has been especially worked at by Rohon, 

 and many beautiful specimens have been figured by him, so that a 

 considerable advance has been made in our knowledge since Pander, 

 Eichwald, Huxley, Lankester, and Schmidt studied these most 

 interesting primitive forms. 



All observers agree that the head-region of these fishes was 

 covered by a dorsal and ventral head-shield, while the body-region 

 was in most cases unknown, or, as in Eichwald's specimens, and in 

 the specimens figured in Lankester and Smith Woodward's memoirs, 

 was made up of segments which were not vertebral in character, but 

 formed an aponeurotic skeleton, being the hardened aponeuroses 

 between the body-muscles. This body- skeleton, which possesses its 

 exact counterpart in Ammoccetes, will be considered more fully when 

 I discuss the origin of the spinal region of the vertebrate. 



Of the two head-shields, ventral and dorsal, the latter is best 

 known and characterizes the group. It consists of a dorsal plate, 

 with characteristic horns, which in Thyestes verrucosus (Fig. 128), as 

 described by Eohon, is composed of two parts, a frontal part and an 

 occipital part (occ), the occipital part being composed of segments, 

 and possessing a median ridge — the crista occipitalis. In Lankester's 

 memoir and in Smith Woodward's catalogue, a large number of known 

 forms are described and delineated, and we may perhaps say that in 

 some of the forms, such as Eukcraspis pustulifcrus (Fig. 127, B), the 

 frontal part of the shield only is capable of preservation as a fossil, 

 while in Cephalaspis (Fig. 127, A) not only the frontal part but a portion 

 of the occipital region is preserved, the latter being small in extent 

 when compared with the occipital region of Auchenaspis (Thyestes). 

 Finally, in Tremataspis and Didymaspis, the whole of both frontal 



