RELATIONSHIP OF AMMOCCGETES TO OSTRACODERMS 327 
Of these, the first two orders belong to the Upper Silurian, while the 
third is Devonian. 
THE DorsaAL HEAD-SHIELD OF THE OSTEOSTRACI 
Of the three orders above-named, the Heterostraci and Osteostraci 
are the oldest, and among them the Cephalaspide have afforded the 
most numerous and best worked-out specimens. At Rootzikill, in 
the island of Cisel, the form known as Thyestes (Auchenaspis) 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 Rohon, 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 Hukeraspis pustuliferus (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 
