ARTHRODIRA. 67 



of the ventro-lateral plates of the trunk there is observed a pair of 

 clavicle-shaped elements (fig. 51, i.l.), but no conclusive evidence of 

 pectoral fins has been noted. There are, however, traces of pelvic fins 

 (fig. 62), each supported by a stout basal cartilage. The hinder ab- 

 dominal and caudal regions are destitute of armour, the only dermal 

 calcification occurring in a narrow band along the lateral line. The 

 notochord must have been persistent, but the stout arches are all super- 

 ficially calcified, of the form and proportions shown in fig. 52. The 



FIG. 52. 



Coccosteus decipiens ; restoration, lateral aspect, about one-quarter nat. size. 

 L. Old Red Sandstone ; N. Scotland. The heterocercal caudal fin is hypo- 

 thetical ; the terminal fin may have been diphycercal. (From Brit. Mus. 

 Catal.) 



dorsal fin-supports are equally calcified, but the fin itself appears to 

 have been membranous. A problematical small plate, showing promi- 

 nent concentric lines of growth, is also conspicuous in the abdominal 

 region immediately behind the much-elongated haemal arches, and may 

 have been an internal element of support in the vertical septum between 

 the right and left halves of some paired organ. The genus Coccosteus 

 occurs in the Devonian both of Europe and America, and all the species 

 are of comparatively small size, none probably exceeding half a metre 

 in length. The typical and best-known species (G. decipiens} is met with 

 in a fine state of preservation in the Old Red Sandstone of Banff, Nairn, 

 Cromarty, Ross, Caithness, and the Orkney Isles. 



Homosteus (fig. 53). Often erroneously quoted (after Hugh Miller) 

 as Asterolepis. The head-shield in this genus encircles the orbits, and 

 the jaws appear to be toothless. The body-shield is comparatively short, 

 and clasps the head-shield on either side by a long, forwardly-directed 

 process outside the articular facette. The orbits (o) are remarkably 

 far forwards, the pineal plate being between their front half; and the 

 so-called occipital plates (e.o., m.o.) are excessively elongated antero- 

 posteriorly. The visceral face of the "median occipital" exhibits cer- 

 tain markings which may denote attachment of the septa occurring 

 between the muscle-plates ; and in that case the brain would be entirely 

 in advance of this plate ; the shield would, indeed, represent much more 



52 



