210 Bashford Dean Memorial Volume 



Sub-class ARTHRODIRA 



The condyle of the neck joint is on the body; the- fossa condyli on the head. The 

 single plates and sensory canals are arranged on the same plan in all forms. The eyes are 

 on the side, the pineal opening on the top, and the nostrils in front of the relatively large 

 head. The gnathal elements are represented by three pairs of plates. In opening the 

 mouth, the whole head moved upward and the movement of the lower jaw was relatively 

 small. The jaws are armed with tooth-like tubercles, cutting edges or crushing 

 plates. The gill arches are in the hinder lower corner of the head; the gill openings be- 

 tween the head and body carapace. 



The Arthrodira range from Lower to Upper Devonian. 



The division of the Arthrodira into smaller groups is a difficult problem, as the 

 members of the whole sub'class are very similar and a great many of the forms are very 

 imperfectly known. The proposed divisions must therefore be regarded as only provi' 

 sional. The first order, without doubt, must comprise these forms, already known from 

 the lower Devonian (Spitsbergen), which show many primitive characters. The detini' 

 tion is: 



Order ACANTHASPIDA 



Arthrodira with a relatively small head and long, narrow body carapace. The 

 gnathal elements are unknown. The distance between the joints is short; the fissure 

 between the head and body armor small. The process on MD is weak. The contact line 

 between the dorsal and ventral carapace is long. The spinal is long and strong. No traces 

 of paired fins are known. The body in some forms is covered with small scales. 



The definitions of the various families and the genera in this order can be found in 

 Broili (1929-1930) and Heintz (1929.1, .2). It inust be noted that the names proposed by 

 Heint2; for the families, I. Monaspidae, II. Mediaspidae, and III. Polyaspidae, must be 

 changed respectively to I. Jaekelaspidae, II. Phlyctaenaspidae, and III. Acanthaspidae — 

 as corresponding better with the international nomenclature regulations. 



The next equivalent group is: 



Order COCCOSTEIDA 



Arthrodira with the head and body carapace of nearly the same size. The gnathal 

 elements are armed with tooth-like tubercles and cutting edges. The distance between 

 the neck joints is large, as is also the fissure between the head and body. The keel on MD 

 is strongly developed. The spinal is more or less reduced. The contact line between the 

 dorsal and ventral carapace is short. The spinal column shows well calcified neural and 

 haemal arches. The supports of the dorsal fins are also calcified. Calcified pelvic girdle 

 and pelvic fins are present. The existence of pectoral fins is uncertain. 



This order can be divided into families and genera as follows : 



1 . Family Coccosteidae with two genera : Coccosteus and Dinichthys. 



2. Family Solenosteidae with genera: Solenosteus, Stenognathus, Machaerognathus, 



Copanognathus, and others. 



