518 THE FORMS OF TISSUES [ch. 



in hand, how far can we go towards depicting the carapace? The 

 five plates of the median row must alternate with their lateral 

 neighbours, and four plates in each lateral row will, accordingly, be 

 the simplest case or most probably number. If at each three-way 

 junction between median and lateral plates the angles tend to 

 equality, it follows that the median plates become converted into 

 more or less regular, or at least symmetrical, hexagons. As to the 

 twenty-four marginal plates, let us put one in front* and one 

 behind, leaving eleven for each side; and let us see to it carefully 



Fig. 198. Asterolepis: an Old Red 

 Sandstone fish. After Traquair. 



Fig. 199. Horny carapace of a 

 tortoise ; diagrammatic. 



that the sutures between these do not coincide but alternate 

 with those of the lateral row. Here we begin to meet with con- 

 ditions of restraint analogous to those of the surface layer of a 

 froth, for the long marginal cells must remain marginal, and their 

 sides must continue to run more or less parallel, or more or less 

 perpendicular, to the edge of the shell; »only in the immediate 



* The Old World tortoises have twenty-five marginal plates, those of the 

 New World lack the anterior median, or "nuchal" plate. This difference is a 

 biological accident, it has neither mathematical interest nor functional significance; 

 it exemplifies the aphorism that whatsoever is possible Nature will sooner or 

 later do. 



