The Structure of Dinichthys 207 



to Stetson's investigations, the nasal capsule in Arthrodires is like that in Elasmobranchs, 

 Concerning the second point, we can only say that the course of the sensory canal system 

 in the Arthrodira can not be compared in detail with that of the Elasmobranchs. In my 

 opinion, if we recall the fundamental differences between Arthrodires and Elasmobranchs, 

 obvious in their whole structures, these two points are not of any great significance. 



THE ARTHRODIRA AS AN INDEPENDENT CLASS 



The problem is — Are the Arthrodira agnathous or gnathostome forms? In other 

 words, are their "lower jaws" homologous or only analogous to the lower jaws of other 

 vertebrates? Let it be stated at once, that the proven facts give no absolutely certain 

 answer to this question. 



As known, it was Dean and Hussakof who especially defended the opinion that the 

 Arthrodira are representative of an independent class, called Arthrognathi, whose jaws 

 are only dermal plates, not modified gill arches as in all other fishes. In favor of this 

 opinion the following facts can be mentioned : 



1. The macroscopical and microscopical structure of the gnathal elements and of 

 the dermal plates in Diyiichthys are absolutely identical. 



2. Dinichthys lincolni CI. has on ASG very distinct tuberculations which are char' 

 acteristic onlv of dermal bones. 



3. In correspondence with all other dermal plates, on the gnathal elements there 

 are never found traces of any kind of enamel or dentine. The "teeth" of Arthrodira, as 

 also the cutting edges, are formed of true bone. 



4. No traces of Meckel's cartilage have ever been found in the Arthrodira. 



5. If one can speak of homology between the infero-gnathal and the dental or 

 splenial, then without doubt the post-sub-orbital can not be regarded as homologous with 

 the palato^quadrate or quadrate in other fishes. It is a typical dermal bone with definitely 

 developed sculpture. 



6. The whole jaw mechanism of the Arthrodira is constructed without the least 

 resemblance to that of any other animal. 



7. The symphysial connection of the infero-gnathal is also of an unusual type. 



8. No traces of ossified hyoid arches or gill arches are known. In the majority of 

 other fishes with such strongly ossified jaw elements, the other components of the 

 visceral skeleton are also more or less ossified. This circumstance is still more remarkable 

 if we remember that the gill arches in the Arthrodira (if present at all) represent a rela- 

 tively highly developed type. 



9. The relative position of the dermal cranial bones is difficult to homologise with 

 the arrangement of cranial bones in other fishes. 



All these arguments are of great importance and make Dean's theory quite probable. 

 But there are other and contradictory facts which may be stated as follows: 



1. In reality, the infero-gnathal was placed comparatively deep under the skin. 

 Especially its hind corner stuck deep underneath the dermal plates (SO and PSO). 



