ACANTHODII. 35 



of Commentry, France, in the Lower Permiau Coal Measures of Germany 

 and Bohemia, and in the Lower Hawkesbury Formation of New South 

 Wales ; but detached spines and teeth indistinguishable from those of this 

 genus occur also in the Coal Measures of North America and in the lower- 

 most Carboniferous strata of Scotland, while a few teeth have been found 

 even in the Upper Trias (Keuper) of Somersetshire. 



ORDER 3. ACANTHODII. 



One large and characteristic group of Palaeozoic Elasmo- 

 branchs is remarkable on account of the presence of membrane- 

 calcifications in the region of the skull and pectoral arch. So 

 far as hitherto examined, however, all these calcifications consist 

 merely of vasodentine or of structureless lamellae without bone- 

 cells ; and the degree of their development is very variable. 

 The group may be conveniently regarded as an order, which 

 commonly bears the name of ACANTHODII, so-called from the 

 first-described genus, Acanthodes (fig. 27). 



FIG. 27. 



Acanthodes wardi ; lateral aspect, one-third nat. size. U. Carboniferous ; 

 Staffordshire. The orbit is represented somewhat too small, and the frills 

 of the gill-clefts are hypothetical. (From Brit. Mus. Catal.) 



The endoskeletal cartilage in the Acanthodians is more or 

 less permeated with granular calcifications, though the supports 

 of the fins are apparently never preserved. The notochord 

 must have been always persistent, while the neural and haemal 

 arches are long and slender, only thickened at the base of the 

 lower lobe of the tail. The membrane-calcifications sheathing 

 the pterygo-palatine and meckelian cartilages are best developed 

 when the teeth are large ; the latter being firmly fixed to these 

 plates. There is a distinct ceratohyal fringed with filaments. 



32 



