Dr. R. H. Traquair — On Chondrosteiis acipenseroides. 257 



The limits of the present paper hardly permit my entering into 

 detail as to the rest of the structure of Chondrosteus. So far as the 

 internal skeleton is concerned, its remains indicate a structure very 

 similar to that in Acipenser, while the fins in shape and arrangement 

 much resemble those of Polyodon. 



Conclusion. — Although there is no evidence of any long snout, 

 Chondrosteus resembled Polyodon in the general shape of the body, 

 in the form and arrangement of the fins, and, above all, in the absence 

 of scales on any part, save the prolongation of the body-axis along 

 the upper lobe of the very heterocercal tail. In other respects its 

 affinities are more with Acipenser. 



How like the corresponding parts in Acipenser are the suborbital 

 bone, the edentulous jaws, the jugal bone, and indeed the palatal appa- 

 ratus, though that has also its own peculiarities ; while it seems highly 

 probable that the mouth was protrusible as in the living Sturgeon. 



But where the resemblances to Acipenser become weaker, they come 

 to point in another direction, namely, that of Palceonisciis, and of 

 course through the Palaeoniscidse to the truly " teleosteoid" Ganoids. 

 This is in the first place well seen in the cranial shield (Fig. 1) 

 where the parietals and frontals are mesially in contact with each 

 other for their whole length, where there is a well-marked supra- 

 temporal chain, and where the post-temporal is moveably articulated, 

 — whereas in Acipenser the frontals are entirely separated by an inter- 

 calated plate, and the post-temporals and two of the median body- 

 plates are iramoveably joined to, and form a part of the cranial 

 buckler itself. Especially palgeoniscoid is, however, the aspect of 

 the opercular and branchiostegal apparatus, as will be seen if the 

 reader will compare the restored drawing (Fig. 5) with the figures 

 of the heads of Palceoniscus and Nematoptychius given in my account 

 of the PalfBoniscidse, although in Chondrosteus there is no prseoper- 

 culum and the series of branchiostegal rays of the two sides may not 

 have met in the middle. The special resemblance of the shoulder- 

 girdle to that of Palceoniscus is also very striking, especially in the 

 form of the post-temporal and supra-clavicular bones. 



In my already-quoted essay on the structure of the Pal£eoniscid£e, 

 I pointed out certain strange and previously unrecognized resem- 

 blances which Palceoniscus bore to Polyodon, especially in the internal 

 skeleton, the shoulder-girdle, and the jaws and palato-quadrate 

 apparatus (even although there are premaxillary bones and there is 

 no evidence that the palato-quadrate elements met in the middle line 

 in front) . I also remarked that the resemblances between Palceoniscus 

 and Acipenser are of course much less prominent. Here, however, 

 is a form which in many of its features presents strong Palceoniscoid 

 resemblances, but whose aflSnities are, nevertheless, more with Aci- 

 penser than with Polyodon ! 



The affinities of Chondrosteus seem, therefore, to radiate in three 

 directions, towards Acipenser, towards Polyodon, and towards the 

 Palseoniscidge, and certainly, of all the three directions, the distance 

 towards Acipenser is the least. 



DECADE III. — VOL. IV. — NO. TI. 17 



