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 _Eukeraspis 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 {328}and occipital region is capable of preservation, the line of demarcation between these two regions being well marked in the latter species. [Illustration: FIG. 127.--A, DORSAL HEAD-SHIELD OF CEPHALASPIS (from LANKESTER); B, DORSAL HEAD-SHIELD OF KERASPIS (from LANKESTER).] In the best preserved specimens of all this group of fishes a frontal median orifice is always present; it appears in some specimens obscurely partially divided into two parts. Perhaps the best specimen of all was obtained by Rohon at Rootziküll, and is thus described by him:-- The frontal part of the dorsal head-plate carried (Fig. 128) the two orbits for the lateral eyes (_l.e._), a marked frontal organ (_fro._), and a median depression (_gl._), to which he gives the name parietal organ. The occipital part (_occ._) was clearly segmented, and carried, he thinks, the branchiæ. I reproduce Rohon's figure of the frontal organ in Thyestes (Fig. 129); he describes it as a deeply sunk pit, divided in the middle by a slit, which leads deeper in, he supposes, towards the central nervous system. [Illustration: FIG. 128.--DORSAL HEAD-SHIELD OF _Thyestes (Auchenaspis) verrucosus_. (From ROHON.) _Fro._, narial opening; _l.e._, lateral eyes; _gl._, glabellum or plate over brain; _Occ._, occipital region.] {329}A similar organ was described by Schmidt in Tremataspis, and considered by him to be a median nose. Such also is the view of Jaekel, who points out that a median pineal eye exists between the two lateral eyes in this animal, as in all other of these ancient fishes, so that this frontal organ does not, as Patten thinks, represent the pineal eye. The whole of this group of fishes, then, is characterized by the following striking characteristics:-- 1. Two well-marked lateral eyes near the middle line. 2. Between the lateral eyes, well-marked median eyes, very small. 3. In front of the eye-region a median orifice, single.