626 L. C. MIALL ON THE STRUCTURE OF THE SKULL OF RHIZODUS. 



this a small prismatic bone, probably a carpal bone, and a fragment 

 of an elongated enamelled bone, probably a fin-ray, can be distin- 

 guished. Behind the cranium is a confused mass of scales ; on a few 

 of these a pattern, not unlike that of Bhizodus, is obscurely seen. 



The features above described show plainly that Bhizodus is a 

 Ganoid fish, and suggest that its place in the Order is not far from 

 Megalichthys. The present specimen does not, it is true, present 

 the most distinctive characters which the hard parts of a Ganoid 

 may yield in a fossil state. The jugular plates are wanting ; and 

 there is no trace of fins or tail. Messrs. Hancock and Atthey have, 

 however, described the jugular plates of the genus on what seems 

 satisfactory evidence. According to their account, there are a me- 

 dian and two lateral plates covered with " strong vermicular sculp- 

 ture, composed of hollows and ridges " *. There is therefore adequate 

 ground for admitting Bhizodus to the Crossopterygian suborder. Its 

 teeth are generally similar to those of Megalichthys, and are of two 

 sizes — the larger being coarsely folded at the base, and dendrodont. 

 It will be interesting to ascertain, whenever an opportunity shall 

 offer, whether Bhizodus, like Megalichthys, has aggregated vomerine 

 teeth. 



A pectoral arch, attributed to this genus, is, or has been, pre- 

 served in the Edinburgh Museum. It is briefly noticed by Dr. John 

 Young in the paper cited below. A more important specimen from 

 the Gilmerton ironstone has lately been described by Dr. Traquair. 

 This reveals " a great part of a well-marked clavicle, resembling in 

 shape that of Holoptychius, and ornamented externally by reticu- 

 lating ridges, furrows, and pits." There is also visible an inter- 

 clavicle (?) and a pectoral fin. Systematists attach considerable 

 importance to the condition of the paired fins in a fossil Crossoptery- 

 gian fish. In this suborder two types of paired fin occur, the one 

 (" acute-lobate ") having an elongate and pointed patch of small 

 scales along the middle of the fin, while in the other (" obtuse- 

 lobate ") the scales are limited to the base. This difference in ex- 

 ternal character is believed to correspond with a noteworthy dif- 

 ference in the solid framework of the limb. Recent fishes with 

 acute-lobate fringed fins (Ceratodus, Lepidosiren) have them sup- 

 ported by an axial skeleton, whereas Polypterus, the sole living 

 example of an obtuse-lobate Crossopterygian, has the fin-rays 

 supported, as in Elasmobranchs and Teleostei, by ossicles arranged 

 in transverse series. Dr. Traquair finds that in Bhizodus the pec- 

 toral fin is obtuse-lobate; and this important character serves, to 

 separate the genus from Holoptychius, with which it has almost 

 uniformly been associated, while the affinity of Bhizodus to Me- 

 galichthys is strengthened thereby. 



Leaving for the present all questions as to the value and compa- 

 tibility of the suborders Dipnoi and Cross opterygii, the genera 

 hitherto included under these names may be arranged in some such 

 way as follows. While so much remains to be done in elucidating 



* Natural-History Transactions of Northumberland and Durham, vol. iii. 

 pp. 83, 84 (1869). 



