THE AMBULACEA OF POSSIL ECHTNOIDEA. 4^1 



separation of successive plates, so as to produce the characteristic 

 plating around the mouth seen in recent forms of Cidaris. 



In the other families of the regular Echinoidea the addition of 

 new plates also occurs at the edge of the radial plate : but there is 

 a crowding of the plates more or less at the peristome, and the oldest 

 plates there gradually become absorbed at the very edge. So that 

 with the growth of the whole test in height there is a superabundant 

 growth of the ambulacra! plates, and, as Loven has well shown, 

 there is a downward or actinai movement of the ambulacral plates 

 from the radial plate to the peristome. This movement is compli- 

 cated by the fact that the ambulacral plates are all growing with 

 the rest of the test, and enlarging in all directions at the surface. 

 Moreover the growth -rate of some plates is greater than that of 

 others, and those which carry tubercles appear to have a greater 

 growth superficially than others. Consequently irregular pressure 

 is exerted by these plates on their neighbours, and the result is very 

 remarkable. Again, the movement of the plates above the ambitus 

 of the test, although comparativel}'- free, is of necessity diminished 

 near the peristome, in consequence of the more or less rigid state of 

 the peristomial region incident to the position of the auricles of the 

 jaws and their processes. 



It is to these different facilities for and oppositions to a regular 

 and symmetrical growth that the varied shapes and characters of 

 the ambulacra and their plates are due. 



It is an interesting and highly suggestive truth that aU the 

 regular Echinoidea should have their most radially situated plates 

 in the form of the simple primaries of the Cidaridae ; but at different 

 distances from the radial plate modifications begin to be seen, and 

 they are characteristic and of generic and specific value. 



The modifications which were known and which had been care- 

 fully described by Loven before the publication of the " Eossil Echi- 

 noidea of Sind," in the ' Palseontologia Indica,' ser. xiv., were those 

 which characterize the Echinidse and some other forms, such as Stron- 

 gylocentrotus (fig. 1). In these the growth -pressure develops com- 



pound plates by jamming and uniting the original primaries; moreover 

 the part of the primary remote from the poriferous portion is often 

 prevented from growing, or is absorbed by the growth-pressure. The 

 result is the formation of demi-plates which do not reach the median 

 line. Moreover the downward growth and the other varieties of 

 growth-expansion cause combinations of several plates, and produce 

 compound geometrical forms made up of three, four, five, and even. 

 * For the explanation of this and the following figures, see pp. 451, 452. 



Q. J. G. S. No. 163. 2 g 



