206 Bashford Dean Memorial Volume 



organs, that the majority of the Arthrodira have the sclerotic rings, which are absent in 

 Macropetalichthys; that the opening for the ductus endolymphaticus is known in Epipet' 

 alichthys but has never been observed in any Arthrodiran; and that an opening (or a deep 

 impression) in the dermal bone for the pineal organ is always found in the Arthrodira, 

 but never in Macropetalichthys . 



If we recapitulate all the previously mentioned facts we find that Macropetalichthys 

 had: (1) the dermal plates on the head placed differently from the Arthrodira; (2) had a 

 different course for the sensory canals; (3) had no body carapace; (4) no joint articulation 

 between head and body; (5) had an un'Arthrodiran'like arrangement of the jaws; (6) 

 had developed no plates corresponding to the side plates of the Arthrodire head; (7) 

 possessed no sclerotic rings; (8) had no opening for the pineal organ in the dermal bone; 

 (9) but in some forms had openings for the ductus endolymphaticus, unknown in the 

 Arthrodira. We also find that no intermediate forms between these two groups 

 are known. 



The resemblance between Macropetalichthys and the Arthrodira is thus based on 

 mere facts that they are all fish-like vertebrates with the head covered by dermal 

 bones, and that all lived during the Devonian Era. These facts, however, are not suf- 

 ficient to prove them closely related forms. 



As known, Stensio thought also that the Ptycodontids (Rhyncodonts) and Jagorinids 

 were nearly related to the Arthrodira. I have already pointed out that the relation 

 between Rhynchodonts and Arthrodires is of a very doubtful character. As to Jagorina 

 — a form from the Devonian of Wildungen, without dermal head roof or side plates on the 

 head, whose body carapace according to Jaekel resembled that of the Chondrostei, and 

 which had numerous small, independent Selachian-like teeth — it is even more difficult 

 than in Macropetalichthys to find any resemblance to the Arthrodira. 



My opinion is that the Arthrodira are not related to any of the above mentioned 

 forms, and that therefore the conclusions drawn in studying these forms cannot be trans- 

 ferred to the Arthrodires. 



If we now analyse the facts given by Stensio (1925, p. 187) to show the relation 

 between the Arthrodira and the Elasmobranchs, we shall see that but few hold good. 

 The points based on the likeness between the structure of the neurocranium and of the 

 brain are of no value for the Arthrodira, as the brain and neurocranium of these forms are 

 too badly known. The blood vessels mentioned are also unknown in the Arthrodires. 

 The position of the palato-quadrate can not serve as a proof, as the Arthrodira had no 

 palato-quadrate at all. The dentition of Elasmobranchs and Arthrodires is as unlike as 

 possible; the Arthrodires had no real teeth, hence enamel and dentine are unknown in 

 their composition.'' Their teeth-like tubercles on the jaws are made of real bone. 



There are then left only two points; the position and relation of the olfactory capsule 

 and the course and development of the sensory canal system. As to the first, according 



" Stensio is of another opinion. 



