New Data on the Internal Anatomy of the Heterostraci (Agnatha), with General Remarks on the Phytogeny of the Craniota

Some new impressions have been observed on the visceral surface of a dorsal shield of Ctenaspis sp. from the Lower Devonian of Spitsbergen. They indicate the probable position of the ophthalmicus profundus and o. lateralis nerves, and of two branchial nerves. The impressions left by the branchial arches are also visible on the same specimen. Furthermore, a median dorsal series of circular impressions previously observed in Seretaspis zychi StensiÖ is compared to similar impressions observed in two osteostracans, and it is suggested that vertebral elements were present in the Heterostraci. Finally, the affinities of the Heterostraci are discussed and it is suggested that they may represent the sister group of the Myopterygii (Cephalaspidomorphi + Gnathostomata).