CYCLIC. OSTRACODERMI. 3 



ORDER 3. CYCLLE. 



Much more satisfactory are some small skeletons from the 

 Lower Old Red Sandstone of Scotland, which make known an 

 extinct order differing from the two existing orders in the 

 remarkable degree of calcification of the skeleton. All the 

 cartilages seem to have been calcified, and vertebral centra are 

 represented by a series of broad rings in the notochordal 

 sheath. The family of PalaDospondylidae (with naked, eel- 

 shaped body, and supposed unpaired narial opening encircled 

 by barbels) is represented by a single genus. 



Palseospondylus (fig. 1). The general aspect of the skull, so far as 

 recognizable, closely agrees with that of a recent lamprey, and there is 

 no evidence of jaws or separate ossifications. When viewed from below 

 (fig. 1) the basicranial cartilage is shown to be continuous, without 

 fontanelles ; and the skull terminates in front in a ring of calcified cirri 

 (d.c., I.e., v.c.) surrounding a large bilaterally-symmetrical orifice (ri), which 

 seems best interpreted as that of an unpaired nasal organ. Behind the 

 skull there extends backwards a pair of undetermined elongated cartilages 

 (x), usually lying in the fossil parallel with the vertebral' column. The 

 vertebral rings are spaced in this (presumably the branchial) region, but 

 throughout the rest of the axis they are closely apposed. There are short 

 and stout neural spines in the abdominal region, but no ribs ; the neural 

 and haemal spines in the caudal region are very slender, while the former 

 are longer than the latter and distinctly bifurcate at the distal end. One 

 known specimen has been supposed to exhibit faint indications of a series 

 of fin-supports on either side of the vertebral column in the anterior 

 abdominal region ; but the precise nature of these appearances cannot yet 

 be determined. A single species, P. gunni, is found in the Caithness 

 Flagstones at Achanarras, near Thurso. The largest specimens are about 

 0'05 m. in length. 



Sub-Class 2. Ostracodermi (or Ostracophori). 



In their fossilized state none of these organisms exhibit the 

 slightest trace of ordinary jawsy or a segmented axial skeleton 

 in the trunk, or arches for the support of paired limbs. Median 

 fins, however, a?e~ present. The head and trunk are invested 

 with a dermal armour remarkable for the extent to which 

 vascular spaces are developed in its middle layer. There is 

 always a shield covering the dorsal aspect of the head, usually 

 followed by a similar shield over the abdominal region ; and 

 opposed to the latter is a ventral armature which meets the 



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