38 



BRITISH FOSSILS. 



On the oth(3r hand, however, it must be considered that, — 



1. The Acanthodidse, unlike all Elasmobranchs, liave great spines 

 articulated -with the pectoral arch. 



2. The dermal plates of the Ganoid Cheirolepis are very shagreen- 

 like, though affirmed by Pander to differ in structure from those 

 of Acanthodidse. 



3. The cranial bones become less and less developed in the 

 Chondrosteous Ganoids, until in Spatularia they are very thin 

 squamose lamellae ; so that there is no great difficulty in the way of 

 supposing their entire absence in a true Ganoid. 



4. In the same way, the opercular apparatus, small in Acci'penser, 

 is still more reduced in SpatulaTia. 



5. The thin, curved, toothless mandibles of S'patularia present, 

 perhaps, the nearest analogue to the singular mandibular bones of 

 Acanthodes. 



6. As Roemer has pointed out, Paleoniscus has orbital plates 

 very like those of Acanthodes. 



7. The production of the pectoral arch into long backwardly 

 directed processes in Diplacanthus and Cheiracanthus is the very 

 reverse of an Elasmobranch character, seeing that the like only 

 obtains, so far as I know, in some Siluroids. 



8. Acanthodes is provided with two very long filaments, beset 

 with short lateral branches, which proceed from the region of the 

 mouth, and such oral tentacles are to be found only in Ganoids and 

 Siluroids. 



Under these circumstances the safest course probably is to 

 reirard the Acanthodidse as a distinct suborder of Ganoids. 



The genera Cephcdaspis, Pteraspis, Auchenaspis, and 3fenasp>is 

 certainly form a family by themselves, to which the title of OephA- 

 LASPID^ may be conveniently applied ; but the position of this 

 family is not readily determinable. No one can overlook the curious 

 points of resemblance between the Siluroids, Callichthys and Lori- 

 cariay on the one hand, and Gephalaspis, on the other, while in other 

 respects, they may be still better understood by the help of the 

 Chondrostean Ganoids. Compare, for example, Scap)irhynchus 

 with Gephalaspis, or the great snout of Pteraspis with that of Spa- 

 tularia. I am inclined to place the Cephalaspids provisionally 

 among the Chondrostei, where they will form a very distinct family. 



The affinities of two genera remain for discussion, the one being 

 the well-known Cheirolepis of Agassiz, the other, the new genus^ 



