CEPHALASPID^. 29 



on the orbital portions, longitudinally on the lateral cornua, concentrically around a 

 central point on the disc, longitudinally on the spine : none of these series of striations 

 run into or form part of another series ; each piece is marked off from its adjacent piece 

 by a slight groove on the upper surface, and by a thickened ridge below : the striae are 

 of the same size in young as in old specimens : in young specimens it is not unusual for 

 the rostrum, spine, and orbital portions to be separate from the disc (particularly in the 

 longirostrate species) : the proportionate size of the various pieces of the shield is the 

 same in large as in small specimens. From these facts the inferences are : 1st, that 

 the growth of the scute was not simple, — that is, it did not proceed merely along the 

 margin of the shield as from one centre ; 2nd, that the number of concentric or parallel 

 striae must have been increased on each portion as the scute advanced in size ; 3rd, that this 

 increase in size must have taken place around or along the margin of each piece. Hence, it 

 appears that each portion had a separate growth ; and when quite young the pieces were 

 very possibly (so far as their calcareous substance is concerned) quite distinct ; they united 

 as they advanced in size, and became firmly joined when fully developed by the thickening 

 and fusing of their adjacent margins. 



The inner surface of the shield in Pteraspis is smooth and shining ; for about half its 

 length, however, on the under surface, the rostrum is striated as on its upper surface ; it is 

 then suddenly excavated, and the hollow of the shield commences, with which the soft parts 

 of the Fish were connected. The inflected thickened margin of the shield all round is 

 striated, as also the under surface of the bulging cornua and the posterior spine. There 

 are ridges on the inner surface caused by the anchylosis of the different pieces ; and there 

 are a very few remarkable ridges radiating from the centre of the disc, varying in the 

 different species. Corresponding to the minute circular piece between the rostrum 

 and disc is a deep pit on the inner surface, which occurs in the species of Pteraspis 

 most markedly, and appears also to exist in Cyathaspis (PI. II, fig. 11). I can offer no 

 explanation of this pit and the diverging ridges. 



The restoration of the shield of the genus Pteraspis was first attempted by Professor 

 Huxley ('Quart. Journ. Geol. Soc.,' vol. xvii, p. 165, 1861), who gave an outKne of the 

 scute in the commoner species Pteraspis rostratus. It has been a matter of very serious 

 difficulty to obtain anything like a satisfactory notion of the form of the " cornua " in this 

 genus, since the abundant casts only mislead, and specimens with the test preserved are 

 hard to get. The figures 8 and 9 in Plate VII represent what I have been able to 

 ascertain by breaking up many specimens. The "cornua" are thick solid bodies, which 

 do not diverge from, but merely project above, the thickened margin of the disc. A large 

 oval orifice opens on each side into the substance of the shield just below the projecting 

 cornu. A short passage, partly perforating the cornu, runs obliquely forward from this 

 orifice, opening widely again into the concavity of the shield, quite at its margin, and 

 within the area of its posterior half. Perhaps this pair of holes were connected with the 

 branchial apparatus. The Rev. Hugh Mitchell, of Craig, was the first to point out 



