. The Bashford Dean memorial volume : archaic fishes. Fishes; Sharks; Fishes, Fossil. The Structure of Dinichthys 205. the two groups. The homology proposed by Stensio is only uncertainly probable, espe- cially when we notice that his '"pre-opercular canal" has only been found in a very badly preserved specimen of Macropetalichthys. The absolutely constant arrangement of the canals in the Arthrodira is not like that in Macropetalichthys (Text-figure 91). No traces of the body carapace have ever been found in Macropetalichthys. We must suppose that it was not developed. If developed a
. The Bashford Dean memorial volume : archaic fishes. Fishes; Sharks; Fishes, Fossil. The Structure of Dinichthys 205. the two groups. The homology proposed by Stensio is only uncertainly probable, espe- cially when we notice that his '"pre-opercular canal" has only been found in a very badly preserved specimen of Macropetalichthys. The absolutely constant arrangement of the canals in the Arthrodira is not like that in Macropetalichthys (Text-figure 91). No traces of the body carapace have ever been found in Macropetalichthys. We must suppose that it was not developed. If developed at all, it was certainly not movably Text-figure 91. The head roofs of an Arthrodire (left) and of Macropetalichthys (right), to show the homolo- gies between the single plates according to Sten- sio. The positions of the sensory canals are shown by dotted lines. P, =PtO, P.,=M: S=EB, L, =iPrO. M,, =MB + i right C + i left C+i right PrO+h left PrO; L,=hC: ^3 "aCf. R, P, PM, SO, PSO, and PH in an Arthrodire are without homologies in MacropetaUchthys. M| in Macropethalichthys has no corresponding plate in an Arthrodire. connected with the head roof since no traces of joint sockets on the occiput are known. Thus, Macropetalichthys had no neck joint,—a trait most characteristic for the Arthrodira. This excludes also the possibility that the Macropetalichthyids had the same jaw mechan- ism as the Arthrodirans. Furthermore, the whole complicated visceral skeleton of an Arthrodiran (with its 14 plates) is unknown in Macropetalichthys. I might remark here that in Synosteus, one of the Wildungen Arthrodira briefly described by Jaekel (), the head is secondarily fused with the body carapace. Therefore the head could not be moved in relation to the body, and the whole jaw mechanism must have been different. We may note here that in the first place this form is so incompletely figured and described that it is difficult to draw any conclusions. And in the second place, the whole
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