576 



The Class Ostracophori 



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

 order includes three famiUes. 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 mnrchisoni occurs in the Upper 

 Silurian of England, and the extraor- 

 dinary Cephalaspis dawsoni in the 

 Lower Devonian of Gaspe, Canada. 

 Eukcraspis pnstiilifera has the head- 

 shield very slender and armed with 

 prickles. In the Thyestidcr the anterior 

 bodv-scales are fused into a continuous 

 plate. Tliyestis and Didymaspis are 

 genera of this type. The Odontotodon- 

 tidcr (Tnviiataspida:) have the shield 

 truncate behind, its surface finely 

 punctate, and the piece between the 

 eyes not fixed. Odontotodiis * sdirenki 

 is found in the Upper Silurian of the 

 Island of Oesel in company with species of Thyestes. The 

 Eiiphaiieropidcc are represented in the Devonian of Quebec. 



Order Antiarcha. — The Antiarcha (avri, opposite; apxoi', 

 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 Asterolepidcc, with numer- 

 ous species, mostly from Devonian rocks. The best known 

 genus is Ptcnchthyodes,'t in which the anterior median plate 



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

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

 ing fishes. 



Fig. 360. — Cephalaspis lyelli 

 Agassiz, restored. (After 

 Agassiz.) 



