In the first part of this pa per a description is given of Coccosteus minor Miller, a minute form, which in some respects represents an intermediate stage between Coccosteus decipiens and Phlyctænaspis. The material is preserved in the Royal Scottish Museum, Eddinburgh.
The second part deals with the development of the gnathal elements in Coccosteus decipiens and shows that the position of these elements as proposed by Watson is not correct. The reconstruction of the body carapace is discussed in detail and indicates that the reconstruction proposed by Watson is, likewise, probably not altogether accurate.
Further, a description follows of the skin fragments, the stomach contents and the shape of the tail in C. decipiens.
In the last part the d'scovery of pectoral fin fragments in C. decipiens is described and, in connection herewith, the development of the pectoral fins in Arthrodira discussed.