The Structure of Dinichthys 



137 



PrE'Orbital. — This plate (Text'figures 12, 13, 14 and 15 PrO) constitutes the front 

 angle of the head roof and defines the fore part of the eye impression. The pre'Orbital 

 process forms its most salient point. On the inside it forms the fore part of the side 

 thickenings which are the thicker part of PrO. The thinner part, developed as a small 

 border along the inner margin of PrO, constitutes the anterior section of the neurocranial 

 impression. The small triangular part in the front of the eye impression is strongly 

 thickened. Here may be found the sockets and ridges which serve to combine the head 

 roof with the sub-orbital and post-nasal. PrO overlaps both the pineal and rostral plates 

 and is itself overlapped by C. 



The sensory canal on the outside of PrO begins in front, not far from the limit of the 

 rostral. At first it makes an elegant curve and then continues downward along the middle 

 of the plate to C (Text-figure 121). 



Pineal. — This is the smallest plate (Text-figures 12, 13, 14 and 15 P) of the whole 

 head roof, and is placed between C, PrO and R. It is overlapped by PrO and itself over' 

 laps C and R. It was perfectly described and figured by Dean (1891) so a detailed descrip' 

 tion is not necessary here. However, we shall only point out that the "pineal fontanelle"' 

 is not found very often. As a matter of fact, my investigations show that the deep, 

 conical impression on the inside of the plate, often ends blindly (Woodward 1922). The 

 openings one may find on the pineal plate are chiefly to be attributed to too energetic 

 preparation, and are made artificially. The author could not in a single instance find in 

 the American Museum collection a pineal 

 plate which showed absolutely and un' 

 doubtedly the natural pineal fontanelle. 



Rostral. — The plate (Text-figures 

 12, 13, 14, 15 R and 20) forms the front 

 part of the top of the head roof. Its front 

 margin is a little thickened on the inside, 

 with two weak impressions for the nostrils 

 on the sides (Text-figures 12, 13 and 20 

 ns). Otherwise the rostral is rather smooth 

 both on the outside and inside. It is over- 

 lapped by P and strongly overlapped by 

 PrO, so that on the outside R often has a 

 T-shaped outline. 



We have now finished the description of all the plates in the head roof and shall pass 

 on to describe the side plates of the head. 



Text-figure 20. 



The rostral plate of Dinichthys intermedius Nwb. : 

 a, froni the outside; b, from the inside. 



ns, nasal impression; P and PrO, positions respectively of pineal 

 and pre-orbital plates. 



THE SIDE PLATES OF THE HEAD 



The Single plates of the side part of the head are not so often found as are the plates 

 of the head roof. Some especially are very rare and known only in a few forms. However, 

 it is very probable that new investigations and better material will bring to light some 



