158 JACKSOX AND JAGGAR STUDIES OF MELONITES MVLTIPORUS. 



pleteness. It is noteworthy that at first sight it is a specimen of indifferent 

 quahty, being considerably worn and compressed. By critical study all 

 the areas were made out, as indicated in the table. The first row of inter- 

 ambulacral plates is absent, but the second exists in each area. The 

 ambulacra have 4 columns of plates at the ventral termination in all 5. 

 areas, the characteristic condition at this portion of the test. Three inter- 

 ambulacra, E^ G and i7, have 9 columns of plates, but the other two areas, 

 A and E, have only 8. They might, however, have shown a ninth column 

 added if preserved to the dorsal area where they are wanting. The initial 

 plates of column three in the 5 areas are hexagonal as usual. The other 

 25 terminal plates of columns in the specimen have the regulation pen- 

 tagonal form. Columns 3 and 4 originate in the second and third row 

 respectively in all areas. Initial plate 4 truncates the dorsal border of 

 initial plate 3 as usual. Column 5 originates in tlie fifth or sixth row, 

 column 6 in the ninth or tenth row, column 7 in the twelfth or thirteenth 

 row, and column 8 in the sixteenth or seventeenth row. The ninth 

 column originates in the one area where counted in thetwent3^-third row. 

 The ninth column in area / originates one column too far to the right, 

 but the other 16 odd-numbered columns originate in a median position, 

 with an equal number of columns on either side. Of the 13 even-num- 

 bered columns, 11 originate with one more column on the left than on 

 the right; the other two originate with one more column on the right 

 than on the left. Of the 18 heptagonal plates adjoining terminal pen- 

 tagons, 13 occupy the correct theoretical position. Thus this specimen is 

 very nearly normal as well as very perfect, showing little departure from 

 the ideal type of plate arrangement, excepting in the unusual position 

 of the ninth column in area /. 



The next specimen tabulated (page 166) is also important as showing 

 the arrangement in 5 interambulacra for comparison. This specimen, 

 number 3010, is in the Museum of Comparative Zoology. Each ambu- 

 lacral area has four columns of plates ventrally. All the areas are want- 

 ing in the dorsal portion, as that is firmly bedded in the rock. One 

 interambulacral area, /, shows 8 columns of plates; the other 4 areas 

 show only 7. It is possible, even i:>robable, that an eighth would have 

 been added to some of them if the dorsal portion were visible. Initial 

 plates of column 3 are shown in areas ^7, E and /. All are hexagonal, 

 having their dorsal borders truncated by initial plate 4 of column 4. All 

 the other 20 columns tabulated originate in pentagonal plates. The same 

 numbered column originates in nearly or quite the same row in each of 

 the 5 areas. A close comparison of the row in which columns originate 

 is requested in this table and the preceding one. This will give one a 

 good idea of the definite uniformity of growth maintained throughout 



