196 Bashford Dean Memorial Volume 



supports for a new problematic plate, which "corresponds in position and function to the 

 vomer or the presphenoid of a recent fish." 



No traces or remains of such a plate have been known hitherto. Possibly a bony 

 plate or cartilage served to connect these two processes and formed the roof of the mouth. 

 The high walls running forward and backward from these processes form the natural 

 boundaries of the neurocramum on both sides. Woodward (1922), Stensio (1925), and 

 Stetson (1930) have also pointed out that these ridges are the limits of the neurocramum. 



To reconstruct the hinder part of the neurocranium is a most difficult problem. Only 

 the position of the foramen magnum, a more or less fixed point, can be of any help. In a 

 form like Dmichthys with a relatively very movable head, it is most natural to suppose 

 that the foramen magnum was placed on the movement axis of the head, (that is in the 

 middle of the horizontal line connecting the fossa condyli, Text'figure 35). If not, the 

 occipital joint could not exist at all. Instead, we would have to find a joint on the spine — 

 a very unusual formation. On the other side is the occipital joint, placed below or above 

 the neck-joint axis, so that in moving the head such a spine would be either compressed or 

 stretched and thus counteract the movement of the head. 



The facts make it very probable, that the foramen magnum was placed on the axis of 

 the neck 'joint (Text-figures 83 and 84 fm). The whole neurocranium thus had an outline 

 as shown on Text-figures 83 and 84. It was a flat box, placed in the median depression of 

 the head roof and supported by the lateral consolidated parts of the head root, especially 

 by the neurocranial processes. Somewhat behind the neurocranial process, the neuro- 

 cranium, following the limits of the ''bridges'' (Text-figure 13 CR) becomes narrower, and 

 in the hind part runs along the median part of MB. In the hind part of MB, where the 

 musculi levatores capitis are attached, the neurocranium loses connection with the head 

 roof, and continues backward along the under side of these muscles. Thus, as seen on 

 Text-figures 83 and 84, the neurocranium extends farther back than the head roof. Pos- 

 sibly the impression on the thickened part of plate MC (Text-figure 13 CR, ti) was 

 occupied by the otic capsules (Text-figure 84 oc). 



Naturally a reconstruction of the brain cavity or the brain itself is a very difficult 

 problem, as our knowledge of the neurocranium is too scanty. We only know the posi- 

 tion of the nasal capsules, the tractus olfactorius (Text-figure 84 to), the pineal organ (D) 

 and eyes (£). The position of the labyrinth organ of the ear is not certain. No traces of 

 other nerve canals are known. 



THE SPINAL COLUMN AND FINS 



Our knowledge of the axial skeleton is very modest. The only specimen (an imper- 

 fectly preserved one) showing traces of notochord and of vertebral arches was described 

 by Dean (1896). As in Coccosteus, so also in Dirxichthys no traces of vertebral centra have 

 been found. Only the neural and haemal arches can be seen. They look like those of 

 Coccosteus, but are shorter and more uniform. 



We know somewhat more about the fins of Dinichthys. Not infrequently there are 

 found long narrow crushed bones, which Newberry had already determined as ''fin rays" 



