574 The Class Ostracophori 



a pectoral fin. The three known genera are Thelodus, Lanarkia, 

 and Ateleaspis. In Thelodus the scales consist of a base and 

 a crown separated by a constriction or neck. Thelodus scoticus, 

 Thelodus pagei, and Thelodus planus are found in the Silurian 

 rocks of Scotland. Other species, as Thelodus tulensis of Russia, 

 extend to the Upper Devonian. 



In Lanarkia the large sharp scales have an expanded base 

 like the mouth of a trumpet. Lanarkia horrida and L. spinulosa 

 are found in the shire of Lanark in Scotland. In Ateleaspis 

 (tesselatus) the skin is covered with small polygonal plates. The 

 lateral flaps or possibly fins take the form of flat rhombic sculp- 



FIG. 357. Lanarkia, spinosa Traquair. Upper Silurian. Family Thelodontidce. 



(After Traquair.) 



tured scales. In this genus the eyes seem to be on the top of 

 the head. 



In the Psammosteidce of the Devonian the head is covered 

 with large plates which are not penetrated by the sense-organs. 

 These plates are covered with minute, close-set tubercles, 

 covered with brilliant ganoid enamel and with finely crimped 

 edges. According to Dr. Traquair, these tubercles are shagreen 

 granules which have coalesced and become united to plates 

 formed in a deeper layer of the skin, as in Ateleaspis the minute 

 scales have run together into polygonal plates. These crea- 

 tures have been considered as "armored sharks," and Dr. 

 Traquair regards them as really related to the acanthodean 

 sharks. Nevertheless they are not really sharks at all, and 

 they find their place with the Pteraspis and other longer known 

 Heterostracans. 



The family of Drepanaspida consists of a single recently known 

 species, Drepanaspis gmundenensis, found in a pyritized condition 



