642 GROWTH OF NEW PLATES. 



always exhibits its simple mode of growth, as in several of the fossil genera 

 allied to Phymosoma and the like. In Arbacia the ambulacra! plates above 

 the ambitus are simple : those of the actinal surface are crowded out of 

 their original position and become arranged in arcs. In the Diadematidae 

 (PL VI f. /.;"). Echinometradae (PL /A'.; PI VI. f. m"). and Echinidae 

 (PI. VII.), the younger stages always have simple single ambulacra] plates, 

 and it is from the study of the young that we have come to a correct 

 understanding of the manner in which additional plates are intercalated, in 

 the ambulacra] series, at different points at the same time, by the splitting 

 up of the original plates, ami by the arrangement of adjoining primitive 

 plates into more or less regular arcs of pores. In this way the ambulacra] 

 plates of the two sides of the median lateral line are apparently arranged 

 into two sets, — one large perforated plate on each side of the suture winch 

 carries the primary tubercles, and a second outer set of minute perforated 

 plates, which are arranged in more or less regular arcs, intercalated be- 

 tween the primary plate-. 



The position of these secondary plates is used to distinguish genera, hut. 

 unless taken from the mode of growth of the plates, the apparent arrange- 

 ment of the pores is deceptive, and similar results are obtained by very 

 different modes of growth, by combinations of parts of one arc with parts of a 

 second arc. The simplest mode of intercalation of new plates is that of Echi- 

 nus (PL VI f. >,). in which a small triangular plate is developed between the 

 two larger ones, and thus trigeminate simple arcs are formed. The same result 

 is also readied in Hemipedina by the curving of the pores round the base 

 of the primary ambulacra] tubercles (PL III. f. .>,). the tubercle riding upon 

 two or three of the original plates, the sutures of which can be plainly seen 

 running through its mammary boss. In Mespilia (PL VI f. 1), Salmacis (PL 

 VI. f. o), Temnopleurus (PL VI. f. 5), Toxopneustes (PL VI. f. ..;. 24), and 

 Evechinus (PL VI. f. .jo), the same mode of growth occurs as in Echinus 

 It is very similar in Phymosoma (PL VI f. ..>) and in Diadema (PL VI. f. 15). 

 In one of the groups of the Echinidae, Amblvpneustes (PL VI. f. 26, 27), Ho- 

 lopneustes (PL VI f. 25, 25 a ), and Hipponoe (PL VI f. 28, 29), the original 

 mode of growth is identical with that of Echinus ; but in the subsequent de- 

 velopment, owing to the great number of plates which are rapidly added, 

 more or less regular vertical rows or irregular trigeminate arcs are formed, 

 due to the lateral crowding of the secondary plates. 



In the Echinometradae the primary plates always retain their greater com- 



