OP THE EARLIER VERTEBRATA. 63 



These cranial bucklers, extending from nape to snout, pro- 

 tected the exposed upper surface of the cartilaginous skull, 

 and conformed to it in shape, as a helmet conforms to the 

 shape of the head, or a breast.-plate to the shape of the chest. 

 And as the cartilaginous heads resembled in general outline 

 the osseous ones, the buckler which covered their upper sur- 

 face resembled in general outline the upper surface of the 

 osseous skull. It was in no case entirely a flat plate ; but 

 in every species rounded over the snout, and in most species 

 at the sides ; and so, in order that its characteristic propor- 

 tions might be preserved throughout the various stages of 

 growth in the head which it covered, it had to be formed 

 from several distinct centres of ossification, and to extend in 

 area around the edges of the plates originated from these. 

 The workman finds no difficulty in adding to the size of a 

 piece of straight wall, whether by heightening or lengthening 

 it ; but he cannot add to the size of a dome or arch without 

 first taking it down, and then erecting it anew on a larger 

 scale. In the domes and arches of the animal kingdom, the 

 problem is solved by building them up of distinct pieces, few 

 or many, according to the demands of the figure which they 

 compose, and then rendering these pieces capable of increase 

 along their edges. It is on this principle that the Cystidea, the 

 Echinidse, the Chelonian carapace and plastron, and the skulls 

 of the osseous vertebrata, are constructed. It is also the 

 principle on which the cranial bucklers of the ancient ganoids 

 were formed. * And from the general resemblance in figure 



* In all probability it is likewise the principle of the placoid skull. 

 The numerous osseous points by which the latter is encrusted, each 

 capable of increase at the edges, seem the minute bricks of an ample 

 dome. It is possible, however, that new points may be formed in the 

 interstices between the first formed ones, as what anatomists term the 

 triquetra or Wormiana form between the serrated edges of the 1am- 

 doidal suture in the human skull ; and that the osseous surface of the 

 oerebral dome may thus extend, as the dome itself increases in size, not 



