36 



BRITISH FOSSILS. 



directed outwards from near the base of the skull (perhaps arising 

 chiefly from the parasphenoid), and ends in a free obtuse surface, 

 against which the middle of the hyoraandibolar suspensorium abuts 

 (PI. VIII., fig. 2). Behind the part from which these processes are 

 given off, the opisthotic and exoccipital ossification is continued 

 backwards as a vertical plate, which forms a large part of the pos- 

 tero-lateral walls of the cranial cavity. Between this plate and the 

 processes of the opisthotic there is a deep fossa, floored below by a 

 thin plate of bone (PL VIIL, fig. 4 h) which forms a sort of con- 

 tinuation of the lower edge of the third process of the opisthotic (?) 

 into the base of the skull. 



The lateral " exoccipital and opisthotic " plates do not reach the 

 roof of the skull superiorly, but end in a free edge, posteriorly ; 

 they diminish to a height of not more than one-eighth of an inch 

 above the base of the skull. From the point where it is lowest 

 (PI. YIII., fig. 4 I) the base of the skull rapidly diminishes in 

 breadth, and ends at e, by a free rounded extremity, which is so 

 fractured that its precise shape is not determinable. 



A thin plate of bone (PI. YHI.^ fig. 4 m) forms a low arch over 

 this part of the skull, and is continuous with the basal plate below. 



It is not improbable that this arch may represent an anterior 

 vertebra corresponding with one of those which, in the Ganoidei^ 

 commonly coalesce with the occipital region of the skull. 



The pro-otic bones of opposite sides are not separated by a greater 

 distance than the width of the presphenoidal bone in front and 

 below, but, above, they are somewhat more distant. 



The anterior end of the snout of Macropoma is constituted by a 

 single bone having the form of a triangle with its base downwards, 

 with a convex anterior and concave posterior surface (Plate VII., 

 figs> 3, 4, and 6). The convex face is beset with small cylindroidal 

 teeth, but at the postero-lateral angles of the oral, or lower, margin 

 of the plate, several larger curved and pointed teeth are attached. 



The posterior concave face is smooth, and seems to have played 

 over the ethmoidal cartilage. 



This bone may either represent the premaxillcs coalesced, which 

 is the interpretation that first suggests itself ; or, on the other 

 hand, it may be a vomer, such as would be formed if the vomerine 

 teeth of Lepidosiren "were supported upon a common bony base. 

 In the latter case the premaxillse remain to be discovered. 



The hyomandibular, quadrate and pterygoid elements of the face 

 are represented in Macropoma by a great triangular plate of bone, 

 in which I have not been able to discover any distinct sutures. 

 The outer and upp^r surface of this " pterygo-suspensorial " bone, 

 and its general form, are well shown in fig. 3., PI. VII., while the 

 proper contour of the hinder half of its lower edge is seen in fig. 6 

 of the same plate. The inner surface of an entire pterygo- 

 suspensorial bone is beautifully displayed in No. 4246 of the 

 British Mueeum collection. 



The whole plate is so twisted upon itself that, anteriorly, its 

 surfaces look almost downwards and upwards, while posteriorly, 

 they look inwards and outwards. The anterior angle is compara- 



