154 JACKSON AND JAGGAR — STUDIES OF MELONITES MTJLTIPORUS. 



or right, in exact accordance with the law as worked out in this paper * 

 The ninth column originates in pentagon 9, with a heptagonal plate on 

 its right ventral side, and with four columns of plates on either side, thus 

 taking its correct theoretical position. The same statement holds good 

 for the introduction of the eighth and ninth columns in the left adjoining 

 interambulacrum of the same specimen, as shown in the figure. This de- 

 parture from the rule of having only seven or eight columns is a variation 

 which might be expected in a type of echinoderms which has already 

 such a multiplication of parts. It is a variation in the direct line of pro- 

 gressive ascent in the group to which it belongs, eleven columns being 

 characteristic of Melonlies glganteus (plate 5, figure 21), the next higher 

 species of the genus. 



The introduction of the ninth column is shown in plate 2, figure 6, a 

 specimen which is also included in the table on page 169. In this speci- 

 men (number 3017) the initial plate of the ninth column is rhombic. 

 It has only built a few plates, having originated shortly before the addi- 

 tion of new plates ceased in the life of the individual. A heptagonal 

 plate lies on its right lower side, as usual. 



While in some cases nine columns of interambulacral plates develop in 

 several interambulacral areas of a specimen, it is not infrequent that only 

 one or two areas acquire nine columns, while other areas of the same 

 specimen have no more than eight. This fact is brought out in the tabu- 

 lations of areas which are considered in succeeding pages. 



Imbrication of Plates. 



Having described the typical arrangement of ambulacral and inter- 

 ambulacral plates in Meloaiies maltiporus, it is desired to leave temporarily 

 the consideration of interambulacral areas and take up the matter of 

 imbrication and the genital and ocular plates as described in this and 

 the succeeding sections. 



Imbrication of plates is quite a common feature in Paleozoic echinoids, 

 being most strongly marked in Lepidocenirus, Lejndechinus and allies. In 

 the Melonitidsef the imbrication is very slight. The ambulacral plates 

 have practically perpendicular sides, both laterally and dorso-ventrally 

 (plate 2, figure 5) ; what inclination occurs may be accounted for by 

 the mechanical necessity for inclined edges in thick plates which form 

 an arcuate test. 



In the interambulacrum the adambulacral plates imbricate under the 

 adjacent ambulacrals (plate 2, figure 5) and not over them, as in Lepido- 

 centrus and Lepidechinus. The median columns of plates of the inter- 



* Excepting that the heptagon next pentagon 5 should be on the right. 



t Excluding the genus Lepidesthes and allies, which are placed in a family by themselves, the 

 liepidesthidoe, Jackson. (See systematic table in the succeeding paper under "Conclusions.") 



