AETHEODIHA. 
Genus COOCOSTuUS ? Agassiz. 
[Poiss. Poss. Y. G. R, 1844, p. 22.] 
Syn. Liognathus, J. S. N ewberry, Rep. Geol. Surv. Ohio, yol. i. pt. ii. ^ 
1873, p. 306. ^ 
c “~ /?- y/4> - /Jt/f'y /77,/rKt 
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 tw T o 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 arch. 
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 Horth 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 2 ; 
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 devonischen Systems (St. Petersburg, 
1857). 
2 R. H. Traquair, “ On the Structure of Coccosteus decipiens, Agassiz,” Ann. 
Mag. Nat. Hist. [6] vol. v. (1890), pp. 125-136, pi. x. 
