CEPHALASPIDtE. 17 



Other markings than these regular ridges are to be noticed, as common to these fish- 

 shields. In some of those with an oval shield of simple construction, the frontal margin 

 is a little thickened, and irregularly marked by coarse serpentine groovings. Besides 

 this, an important piece of evidence with regard to the living condition of the scutes is 

 furnished by the presence of small depressions or " pits" arranged in series on the striated 

 surfaces diverging from the centre. These pits bear all the appearance of being the 

 positions or sites of mucous glands, such as are abundant in all Fishes, and from them we 

 may conclude that a secreting membrane was closely attached to the striated calcareous 

 material. 



3. Variations in the Form and Construction of the Shield. — In all the Heterostraci 

 the shield is markedly concavo-convex, the lateral convexity being greatest. The 

 margins are slightly inflected in all,^ and thickened, especially where there is a posterior 

 spine developed, or an anterior rostrum. In the Osteostracous genus Cephalaspis 

 the anterior margin of the shield is inflected to a larger extent and thickened into a 

 well-marked and strong " rim." In Pteraspis the rostrum is thick, strong, and solid, 

 as also the posterior spine and lateral margins forming the " cornua"; they are striated on 

 the under as well as the upper surface, and evidently projected to some extent beyond the 

 softer portions of the animal covered by the curved shield. 



In the simplest genus, Scaphaspis, the shield is composed of a single piece, of an oval 

 shape, usually terminating acutely or in a short spine. In the most complex it is 

 formed by the coalition of an anterior rostrum, two orbital pieces, a large central disc, 

 two lateral perforate cornua,^ and a posterior spine (see fig. 3, page 15). The genus 

 Cyathaspis is intermediate between these two ; it presents a short rostrum, two lateral 

 cornua, and a central disc. By the aid of this form of shield, it is perceived that the 

 anterior transversely marked margin of the shield in Scaphaspis represents the " rostrum" 

 of the two other genera, which in Cyathaspis is much reduced ; at the same time the acute 

 termination of the shield (or spinous tubercle) typifies the large posterior spine of Pteraspis? 

 It is interesting to note the gradual development from the simple to the complex form in 

 these shields (the differentiation accompanied with integration), and it is much to be 

 desired that renewed researches may bring to light additional intermediate forms to 

 complete the connection. 



The pieces making up the complex shields are not actually separate. They form one 

 continuous whole, but are distinctly emarginated by grooves and differing convexity. 



' The lateral cornua in the genus Pteraspis ave greatly thickened and inflected at the margin, forming, 

 indeed, hollow dilated rims to the shield, which are perforated on each side. It is very difficult to find 

 any explanation of these open excavated structures, unless they be " spiracles." See fig. 3, p. 15, and 

 Plate VII, figs. 8, 9, 16, 17. 



2 The separation of the orbital pieces from the lateral cornua in Pteraspis is not quite satisfactorily 

 ascertained. 



' The simple ovoid scutes of Scaphaspis {Cephalaspis Lloydii, &c., of Agassiz) have been sometimes 

 regarded by local collectors and others, as the "torsos" or central portions of the apparently more 

 complete Pteraspis. This idea is, however, clearly shown by well-preserved specimens to be erroneous. 



3 



