250 Dr. R. H. Traquair — On Chondrosteiis acipenseroides. 



nomenclature. In advance of the two parietals are two largo 

 frontals (/.), also in apposition mesially, while posteriorly, and 

 externally, each of them also touches the squamosal of its own side. 

 Then, occupying a corner between the hinder part of the external 

 margin of each frontal and the anterior external margin of the 

 corresponding squamosal, is a rather small posterior frontal {p.f.). 

 In no specimen which I have seen are the plates in advance of 

 the frontals in sufficiently good preservation to enable one to map 

 out or describe them : the outline of the snout as given in the 

 restoration (Fig. 5) is therefore conjectural. Behind the posterior 

 margins of the parietals and squamosals, and between these and the 

 post-temporal elements of the shoulder-girdle, is a transverse row 

 of five small plates (s.t.). One of these, of a rudely polygonal shape, 

 is median, and placed just at the union of the two parietals, and is 

 flanked on each side by another plate, of which the right is larger, 

 the left smaller than itself. External to each of the latter is the 

 remaining plate of each side, of considerably larger size, of an 

 irregularly triangular shape, and placed between the posterior 

 margin of the squamosal and the post-temporal (p.t), which latter 

 it largely overlaps. These are certainly the plates which in other 

 Ganoid fishes (Zepidosteus, Polypterus) have sometimes been called 

 " supra-occipitals " and "epiotics," but they are mere scale-bones, 

 and occupy the place of the supra-temporal chain in Teleostei. 



A very distinct suborbital bone (s.o. Fig. 5) is seen in a large 

 number of specimens. It consists of two limbs — an upper and 

 longer vertical one meeting below at nearly a right angle with a 

 shorter horizontal portion, the bone being considerably expanded at 

 the junction. Above, the suborbital was suspended from the post- 

 frontal region of the cranial shield, — below, it comes in contact with 

 the middle of the maxilla. This suborbital is the bone which Sir 

 Philip Egerton has interpreted as the " prgemaxilla " in his memoir. 



All these plates are externally marked with pores, and often with 

 furrows and ridges radiating from the centres of ossification ; often 

 also the surface becomes corrugated, sometimes almost granulated ; 

 but I have seen no positive traces of ganoine upon the surface. 



Internal Cranial Bones. — There is a large parasphenoid much 

 resembling that of Acipenser in shape ; but, unlike Prof. Young, I 

 can find no remains of ossification in the chondrocranium, which can 

 be described, or even relied upon as being such. Although such 

 ossifications may very likely have existed, it seems very improbable 

 that they attained any considerable dimensions. 



Hyoid Arch. — The most easily recognized bone of the entire head 

 is the hyomandibidar (7j. m. Figs. 2 and 5), which passes from the 

 squamosal region obliquely downwards and backwards. It is shaped 

 much as it is both in Acipenser and Polyodon, being constricted in 

 the middle and flattened anteroposteriorly in its upper part, laterally 

 in its lower. In one specimen in the British Museum there is an 

 appearance as of an ossified symplectic, extending from the lower 

 extremity of the hyomandibular towards the articulation of the 

 mandible ; this I do not insist upon, as it is not corroborated by any 



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