﻿ARTHRODIRA. 



Genus COCCOSTEUS, Agassiz. 



[Poiss. Foss. Y. G. K. 1844, p. 22.] 



Svn. Liof/nathus, J. S. dewberry, Rep. Geol. Surv. Ohio, vol. i. pt. ii. 

 1878, p. 306. 



Head and trunk broad, the dorsal aspect more or less arched from 

 side to side ; scutes ornamented with rounded stellate tubercles ; 

 neural and haemal arches well calcified, and the caudal region des- 

 titute of armour. Elements of cranial shield not fused in the 

 adult, and the occipital bones constituting less than half of its 

 length ; a distinct small median bone over the pineal region, not 

 perforated ; orbits forming broad notches, not bounded externally ; 

 sclerotic ossified ; premaxilla and maxilla distinct, and one or two 

 inner pairs of dentigerous bones in the upper jaw ; mandibular 

 rami suturally united at the symphysis, each bearing a short series 

 of conical teeth anchylosed with the middle of its oral margin. A 

 single median dorsal shield upon the trunk, with an inner longi- 

 tudinal keel, and rounded or acutely pointed posteriorly ; ventral 

 armour of trunk well developed, consisting of two large lateral 

 plates and two small diamond-shaped median elements, the whole 

 shield united with the median dorsal by two dorso-lateral and two 

 truly lateral plates ; anterior dorso-lateral plate with an articulating 

 eminence, but no forwardly directed process. A pair of short deep 

 plates meeting in the median line immediately in advance of the 

 ventral and lateral armour, evidently representing the pectoral areh. 

 A single short median dorsal fin upon the anterior portion of the 

 caudal region, without fin-rays, supported by a double series of 

 robust, superficially ossified cartilages, equal in number to the 

 apposed neural arches. 



This is the type genus of the family, and is more completely 

 known than any of its allies, on account of the fine state of preser- 

 vation in which its remains occur in the Lower Old Red Sandstone 

 of the Xorth of Scotland. Since the researches of Agassiz, Hugh 

 Miller, and Egerton, much information concerning the skeleton of 

 the fish has been obtained and published by Pander 1 and Traquair '; 

 and the accompanying figures and description are chiefly based upon 

 the most recent memoir of the latter author. 



The cranial shield (fig. 42) is irregularly six-sided in shape, the 



1 C. H. Pander, Die Placodermen des devonischeii Systems (St. Petersburg, 

 1857). 



2 R. H. Traquair, " On the Structure of Corcoztcuz dccipitns, Agassiz," Anu. 

 Mag.- Nat. Hist. [GJ vol. v. (1890), pp. 125-13(5, pi. x. 



