262 



COCCOSTEOMORPHI 



many important differences. The real affinities of the Coceosteo- 

 morphi are still obscure. 



The Sub-Class may be divided into two Orders — the Anarthro- 

 dira (Dean [109]) and the Arthrodira. 



Order 1. ANARTHRODIRA. 



A small group of incompletely known fish, perhaps representing 

 a less specialised type than the Arthrodira. Large sculptured 

 plates extend dorsally from the cranial shield over the trunk. 

 There appears to be no differentiated joint between the armour of 

 the two regions, the limit being marked only by an internal shelf of 

 bone. But perhaps the whole shield belonged to the skull, and there 

 was no trunk-armour (Eastman [129]). The lateral-line canals are 

 sunk below the superficial ornamental layer and open by a double 

 series of pores. There is no separate pineal plate, and the frontals 

 are widely separated by the median occipital which meets the ethmoid. 



Family Macropetalichthyidae. Macropetalichthys, Norm, and Owen ; 

 Devonian, N. America and Europe. (? Asterosteus, Newb. ; Devonian, N. 

 America.) 



Order 2. ARTHRODIRA. 



The cranial shield is movably jointed with the cuirass of the 

 trunk. The ' centrals ' (parietals) touch in the middle line, separ- 

 ating, as a rule, a pineal from the median occipital plate. Super- 

 ficial grooves indicate the position of the lateral-line canals. 



Sub-Order 1. ARTHROTHORACI. 



The two joints between the shields are typically developed. 

 Doubtless in correlation with the powerful dentition there is 



generally a con- 

 siderable space be- 

 tween the two 

 dorsal shields 

 above, allowing 

 the cranial shield 

 to be uplifted ; the 

 lower jaw appears 

 to have but little 

 play, being closely 

 followed by the 

 ventral plates. The 

 orbits were bound 

 behind and below 

 l>y a maxilla (sub- 

 orbital). Except in the Coccosteidae, the preorbitals do not exist. 



Fig. 230a. 



( 'occosteus bich nsis, v. Koen. Restoration of head and trunk s 

 /»', pectoral spine ; v.s, ventral shield. (After Jaekel.) 



