57 6 



The Class Ostracophori 



lateral-line channels or sense-organs. Ventral shield single. The 

 order includes three families. The Cephalaspida have the shields 

 tuberculate, the one between the eyes fixed, and the anterior 

 body-shields are not fused into a continuous plate. The best 

 known of the numerous species is Cephalaspis lyelli from the 



Lower Devonian of England. Hemicy- 

 claspis murchisoni occurs in the Upper 

 Silurian of England, and the extraor- 

 dinary Cephalaspis dawsoni in the 

 Lower Devonian of Gaspe, Canada. 

 Eukeraspis pustulifera has the head- 

 shield very slender and armed with 

 prickles. In the Thyestida the anterior 

 body-scales are fused into a continuous 

 plate. Thyestis and Didymaspis are 

 genera of this type. The Odontotodon- 

 tida (Tremataspidce} have the shield 

 truncate behind, its surface finely 

 punctate, and the piece between the 

 eyes not fixed. Odontotodus * schrenki 

 is found in the Upper Silurian of the 



Island of Oesel in company with species of Thyestes. The 

 Euphaneropidcs are represented in the Devonian of Quebec. 



Order Antiarcha. The Antiarcha (avn, opposite; apxov, 

 anus) have also bone-corpuscles in the plates, which are also 

 enameled. The sense-organs occupy open grooves, and the dorsal 

 and ventral shields are of many pieces. The head is jointed 

 on the trunk, and jointed to the head are paddle-like appendages, 

 covered with bony plates and resembling limbs. There is no 

 evidence that these erectile plates are real limbs. They seem 

 to be rather jointed appendages of the head-plate, erectile on 

 a hinge like a pectoral spine. There are traces of ear-cavities, 

 gill-arches, and other fish-like structures, but nothing sug- 

 gestive of mouth or limbs. 



This group contains one family, the Asterolepida, with numer- 

 ous species, mostly from Devonian rocks. The best known 

 genus is PtenchthyodesJ in which the anterior median plate 



* This name, inappropriate or meaningless, is older than Tremataspis. 

 t The earlier name of Pterichthys has been already used for a genus of liv- 

 ing fishes. 



FIG. 360. Cephalaspis lyelli 

 Agassi z, restored. (After 

 Agassiz.) 



