PYGOPTERUS GREENOCKII, ETC. 709 



broader and shorter proportionately. The square-shaped parietals, the more 

 elongated frontals, the mastoids, the post-frontals forming the posterior, and the 

 pre-frontals the anterior part of the orbital margins of the shield, may all be dis- 

 tinctly recognised. The same line passes along the parietal and frontal on each 

 side which I have noticed in Pygopterus. The nasal bone certainly projects for- 

 ward over the mouth, but is invariably so crushed as to render it almost impossible 

 to tell with any certainty if it be a single bone, as in Pygopterus, or double, as in 

 Lepidosteus and Polypterus. For my part, I should incline to the former opinion. 

 The head may now be seen to be very similarly constructed in Pygopterus 

 and in Amblypterus. On comparing the head of Pygopterus with that in the 

 recent Lepidosteus and Polypterus, it will be seen that the gape is still wider, and 

 the articulation of the lower jaw still further back than in the latter genus, so 

 that the suspensory and opercular apparatus are directed obliquely downwards 

 and backwards. 



In its relation to the superior maxillary bone, the orbit is still more anteriorly 

 situated than in Polypterus, being placed over the anterior part of the upper jaw, 

 whereas in Lepidosteus the projection of the snout and the length of the suspen- 

 sorium carry both jaws forward altogether in front of the eye. The superior 

 maxillary bone is simple, as in Polypterus — not divided into a number of pieces, 

 as in Lepidosteus. As in both Lepidosteus and Polypterus, the intermaxillary 

 bone is immovably articulated to the front of the cranium, only a limited 

 amount of motion being allowed to the superior maxillary. There are a greater 

 number of separate bones exposed on the top of the cranium than in Polypterus ; 

 but the nasal bone is single, thus differing from the nasal both in Polypterus and 

 Lepidosteus, which is double. The post-frontal is seen on the surface of the skull, 

 but again it is doubtful whether any representatives of the supra or paroccipitals 

 take part, as in Lepidosteus, in the formation of the cranial shield. The branchi- 

 ostegal membrane was strengthened by rays instead of by two "jugular" plates, 

 as in Polypterus, but again these rays differ from those of Lepidosteus in being 

 many in number, and enamelled on their surfaces. 



The bones of the shoulder-girdle are the same as those in Polypterus. Lastly, 

 the head of Pygopterus differs from that of Polypterus in the absence of the 

 spiracle, and the row of ossicles associated therewith. The only undoubted 

 members of the system of superficial facial bones are the suborbitals and the plate 

 covering the cheek ; the presence of supratemporals is doubtful ; and I have seen 

 no trace of the " os mobile du nez," though this, judging from its small size in 

 Polypterus, may readily become indistinguishable in a fossil specimen. 



Turning now to Eurynotus (Ag.), a genus not uncommon in the Scottish Car- 

 boniferous strata, but rare elsewhere, we find that it has many points of resem- 

 blance to the two fossil genera already noticed, but also very many of decided 

 difference. 



VOL. XXIV. PART III. 9 G 



