16 



OLD RED SANDSTONE PISHES. 



Fig. 



/. 



Thyestes. 



§ VIII. Sectio A. — Heterostraci. 



Before proceeding systematically to describe the genera and species ofHETEROsTRACous 

 Cephalaspid^, illustrated in the Plates, it is necessary to say a few words with regard 

 to the generic divisions given in the Synopsis above, and the resemblances and differences 

 which the groups exhibit. It will be observed that the division into genera is based on 

 the greater or less complexity of construction of the shield, it being stated to be simple in 

 the first genus, composed of four pieces in the second, and of seven in the third. The 

 nature of this separation into pieces is a question of some importance, which must be 

 considered, as well as the limits of variation in ornamentation, and other matters. 



1. Variations in the Intimate Structure. — The variations in intimate structure in the 

 shields of Heterostraci are unimportant. As far as they have been examined in this 

 respect the differences are merely in size, in the thickness of the layers, dimensions of the 

 polygonal cavities, &c., which have been fully described (pages 10 — 12). 



2. Variations in the Surface-markint^s. — The highly characteristic striations of the shield 

 in Heterostraci vary considerably in different species. In all they bear a definite relation 

 in their disposition to the outline of the plate or piece of the shield which they mark, each 

 addition to the size of the plate appearing to result from the addition of new ridges 

 parallel to and representative of those which have preceded. In this way the oval scutes 

 become marked by a concentric series of oval groovings of greater or less regularity, 

 whilst the distinct portions of the scute in Cyathaspis and Pteraspis show a regular 

 consecutive series of transverse or curved markings. The grooves and their interjacent 

 ridges vary in size in different species, from 50 to 200, or even more, going to the inch. 

 The form of the ridges varies ; some species presenting a flat-topped ridge, others a 

 rounded one. In some species the inner sides of the ridges are crenated, in others 

 smooth, whilst in one species there are ridges of large size interstriated by much smaller 

 ones. 



