18 



OLD RED SANDSTONE EISHES. 



Each piece has its own series of striations, and on the smooth concave surface of the 

 shield exhibits an apparent hne of juncture with its neighbouring piece, in the form of an 

 irregular rounded ridge. The pieces are in fact fused at their margins ; they were, there 

 is reason to believe, distinct when the Fish was young ; and specimens of small central 

 discs and rostra of Pteraspis occur unconnected and separate : wlien growth was com- 

 pleted anchylosis took place, and the pieces formed a single united shield. The unmis- 

 takable appearance of each piece having had a separate development and growth 

 exhibited by the concentric markings and irregularities of form in complete shields of 

 Pteraspis, when considered together with the presence of the ridges formed by the fused 

 margins of the different pieces, seems to point to the conclusion that they were once uncon- 

 nected and separated. It may be said, that if these pieces originated by separate develop- 

 ment they would be united, not by a fusion of their margins, marked by an elevated ridge, 

 but by sutures. In reply to this, I must call to mind the peculiar nature and histological 

 structure of the calcified shield ; besides which anchylosis occurs in the head-bones of living 

 Ganoids {Poli/pterus), leaving a straight, elevated ridge as its indication. It does not appear 

 that there is any reason to suppose that the union of separate pieces in Pteraspis would 

 have occurred otherwise than in the manner indicated ; and hence, I believe, I am justified 

 in regarding the demarcated portions of the shield in Cyaihaspns and Pteraspis as separate 

 calcifications. 



4. Scales of Heterostraci. — All that is known as regards the scales of these Eishes is from 

 a single specimen found in the Cornstones of Herefordshire, which shows a few nearly 

 equilateral rhomboid scales, ranged behind the posterior portion of a head-shield. It is 

 probable that all Heterostraci possessed scales of this form, and very possibly others larger 

 as well. 



Fig. 8. Diagram of Pteraspis, showing both Shield and Body . 



5. Hypothetical Form of the Body ; Position of the Mouth, Sfc. — The Woodcut, fig. 8, 

 gives a purely hypothetical view of a species of Pteraspis. It has been introduced 

 to exhibit the probable position of the shield and the form of the body. We h:now 

 absolutely nothing of these matters ; but it appears highly improbable (by the absence of 

 such remains) that the mouth of this Fish was provided with hard teeth, or in fact was 

 anything more than a suctorial organ ; and there is reason to believe that what is known 

 with regard to Cephalaspis as to the shape of the body is true to some extent for the 

 parallel Heterostracous forms. 



