518 Notices of Memoirs— Dr. JR. H. Traquair's Address. 



in the Ehizodontidas they are rounded, deeply imbricating, and 

 normally devoid of the ganoine layer, which, however, occasionally 

 recurs on the scales of Bhizodopsis and the fin-rays of Gyroj^tijchius. 



What, then, of the structure of the paired fins? Fortunately, in 

 the Khizodont genera Tristichopteriis and Husthenopteron the internal 

 skeleton of the lobe was ossified, and what we see clearly exhibited 

 in the pectoral of some specimens is striking enough. We have 

 a basal piece attached to the shoulder-girdle and followed by a median 

 axis of four ossicles placed end to end. The first of these shows 

 on its postaxial margin a strong projecting process, while to its 

 preaxial side, close to its distal extremity, a small radial piece is 

 obliquely articulated, and a similar one is joined also to the second 

 and third segments of the axis. The arrangement in the ventral fin 

 is essentially similar. 



In fact, we have in the Ehizodontidae a short imiserial 

 ' archipterygium,' and the question is, Has this been formed by the 

 shortening up and degeneration of an originally elongated and 

 biserial one, or on the other hand, do we find here a condition in 

 which the stage last referred to has not yet been attained ? This 

 question is inseparable from the next, whether the Rhizodonts or 

 the Holoptychians form the most advanced type. 



The Holoptychiidee resemble the EhizodontidcS extremely closely 

 in their external head-bones, in their rounded, deeply imbricating 

 scales, and in the form and arrangement of their median fins. But 

 the teeth show a more complex and specialized structure than those 

 of the Rhizodontidae ; the simple vertical vascular tubes formed by 

 the repeated folding of the dentine in that family being connected 

 by lateral branches around which the dentine tubules are grouped in 

 such a way as to give rise in transverse sections to a radiating 

 arborescent appearance ; hence the term ' dendrodont.' In this 

 respect, then, the Holop ty child ee show an advance on the Rhizo- 

 dontidge — what then of the paired fins ? While the ventral remains 

 subacutely lobate, as in the previous. family, the pectoral has now 

 assumed an elongated amtely lobate shape, with the fin-rays arranged 

 along the two sides of a central scaly axis exactly as in the Dipnoi ; 

 and though the internal skeleton has not yet been seen, yet, judging 

 by analogy, we cannot escape the belief that it was in the form of a 

 complete biserial ' archipterygium.' 



What, then, is the condition of afiairs in the oldest known 

 Dipnoan ? 



The oldest member of this group with whose configuration we 

 are acquainted is Dipteriis, which likewise appears in the middle of 

 the Devonian period simultaneously with the Osteolepidee, Rhizo- 

 dontidae, and Holoptychiidas. In external form it closely resembles 

 a Holoptychian, having a heterocercal caudal fin, two similarly 

 placed dorsals, one anal, and circular imbricating scales, which, 

 however, have the exposed part covered with smooth ganoine. But 

 now we have the ventral s as well as the pectorals acutely lobate 

 in shape, and presumably archipterygial in structure; the top of 

 the head is covered with many small plates, there is no longer 



