162 JACKSON AND JAGGAR — STUDIES OF MELONITES MULTIPORUS. 



vails in its principles, but varies somewhat from generic differences of 

 structure. 



Summing up, we find that in Melonites multiporus 35 interambulacral 

 areas are figured or tabulated which are nearly or quite entire dorsally. 

 Of these 2 have 7 columns of plates, 18 have 8 columns, and 15 have 



9 columns. Therefore it may be stated that 7 columns is an unusually 

 small number of columns for this species, it usually possessing either 8 or 

 9, with about even chances for either number, but slightl}^ preponderat- 

 ing in favor of 8. Nine columns of plates have never been described as 

 existent in this species, which is somewhat strange, as this number is 

 evidently so frequent. 



The introduction of new columns (succeeding columns numbers 1 and 

 2) is shown in 248 cases in Melonites multiponis and giyanteus. Of these 

 the initial plates of the columns are pentagonal in 220 instances, in every 

 case of which an apex of the pentagon is directed ventrally and a side of 

 the pentagon is directed dorsally. Twenty-two of the terminal plates are 

 initial hexagons of column number 3, which is the normal form in this 

 instance. Only in 6 cases out of the whole number 248 are the initial 

 plates of columns other than the regulation form ; that is, the form of 

 the initial plates of columns is correct by rule in 97+ per cent of the 

 cases. 



In the matter of the position of introduction of columns of Melonites 

 multijporus as expressed by the number of rows from the base in which 

 the column originated, there is for the most part great uniformit3\ Col- 

 umns 1 and 2 always are represented by the two plates found at the 

 ventral termination,* as observed in 21 cases (page 143).t The third 

 column originated in the second row, as shown in 26 cases. The fourth 

 column originated in the third row, as observed in 27 cases ; in one speci- 

 men (plate 3, figure 16) this column originated in the fourth row and in 

 one (plate 3, figure 12) it originated considerably later; but these are the 

 only exceptions that have been seen. The fifth column originates in the 

 fifth row in 3 cases, in the sixth row in 17 cases, in the seventh row in 8 

 cases, and in the eighth row in 7 cases ; therefore the fifth column in the 

 great majority of cases originates in the sixth row. The sixth column in 



10 cases originates in the ninth row, in 18 cases in the tenth row, and in 



*That is, the ventral termination as normally existing in an adult. In this summing up of areas 

 the initial first formed plate 1' (plate 3, figure 10) is not considered. It is a feature properly be- 

 longing to an earlier period of growth and is resorbed by the encroachment of the peristome. We 

 are now considering the adult, and therefore consider the ventral border as it exists in that period 

 of growth. 



tExcepting this statement in regard to the first two columns, all the observations cited are those 

 figured in plates or shown in detail in the tables. Of course many other observations were made, 

 but we limit the remarks to the published observations. 



