?£T- FORE-RUNNERS OF BACKBONED ANIMALS. 599 



189:!. 



which we publish other restored sketches. That the three types, to 

 which reference has been made above, are all closely related and 

 represent three grades in a great extinct group, can scarcely be 

 longer doubted ; and it is now customary to arrange them in a 

 division of undetermined rank and value, termed Ostracodermi 



or OSTRACOPHORI. 



The simplest of these armoured types are Pieraspis and its allies, 

 (Figs. 2, 3, 9), which have the head and abdominal region shielded 

 both above and below, while the tail, in some cases at least, is 

 covered with scales (3, 12). The tissue of the shield is peculiar, each 

 plate consisting of an outer and inner hard layer with a relatively 

 thick middle layer of polygonal chambers, through which the sensory 

 canal-system of the skin forms ramifying delicate tunnels. There 

 are no bone-cells in either layer of the shield, and the external 

 surface is always concentrically striated. The eyes (Fig. 3, orb.) are 

 widely separated, being placed laterally ; and towards the hinder 

 border of the dorsal shield there is an aperture on each side, evidently 

 to allow the water to escape from the gill-cavity. The nostrils must 

 have been placed close to the mouth on the under surface of the 

 head ; but of both mouth and nose nothing is known. 



When the dorsal shield of one of these organisms — especially of 

 Cyathaspis (1,3, 12) — is examined from the inner (or visceral) face, it 

 shows a number of symmetrically arranged impressions, which are 

 also of interest from the possible clue they afford to the arrangement 

 of the soft parts (Fig. 2). A pit in front {pin.) between the ordinary 

 eyes probably lodged a rudiment of the pineal eye ; and a paired 

 series of lateral indentations {hr.) is suggestive of the original presence 

 of well-separated gill-pouches, between which it is reasonable to infer 



EXPLANATION OF PLATE. 



Fig. I. — Palciospondylns gunni ; skeleton, twice nat. size. Lower Devonian, Caith- 

 ness, au. auditory region of skull. /. supposed lip-cartilage of mouth, p. 

 processes, s. dorsal shield, v. ring-vertebra, end-view. 



Fig. 2. — Cyathaspis sturi ; inner view of dorsal shield. Upper Silurian, Galicia. 

 au. supposed impressions of auditory organs, br. impressions of branchial 

 sacs. pin. impression of pineal body. [After Alth.] 



Fig. 3. — Pteraspis rostrata ; upper view of dorsal shield. Lower Devonian, Hereford 

 shire, orb. orbit. [After Lankester.] 



Fig. ^.—Tremataspis schrenki; dorsal and ventral (4a) aspects of shield. Upper 

 Silurian, Isle of Oesel. ant. antorbital sense-organ, orb. orbital opening. 

 p.o.v. postorbital vacuity, x. vacuity (?) [After F. Schmidt.] 



Fig. 5. — Ceplialaspis lyelli ; upper view of dorsal shield. Lower Old Red Sandstone, 

 Forfarshire, n. undetermined rows of pits (seen only on the inner face of the 

 shield), orb. orbit, p.o.v. postorbital valley. [After Lankester.] 



Fig. 6. — Auclienaspis egertoni ; upper view of head and trunk. Lower Old Red 

 Passage Beds, Ledbury, orb. orbit, p.o.v. postorbital valley. 



Fig. y.—Pterichthys milleri ; dorsal, ventral (7a), and lateral (yi) aspects of head and 

 trunk. Lower Old Red Sandstone, N. Scotland, ap. appendages. ;. joint in 

 appendage, m. supposed upper jaw. op. operculum, orb. orbit. [Partly 

 modified after Traquair.] 



Fig. 8. — Acanthaspisdecipiens : ventralaspect of shield. Lower Devonian, Spitzbergen. 

 ap. appendage, pr. process supporting appendage. 



