378 . ROBERT TRACY JACKSON ON ECHINI. 



and three plates in the second row; with very rare exception the fourth cohimn originates in 

 the third row and above this zone the additional columns come in at somewhat definite intervals 

 until the full complement is attained, as worked out by Jackson and Jaggar (1896). Dorsallj', 

 before reaching the apical disc, the adambulacral columns 1, 2, and some additional median 

 columns may drop out as a character of senescence (Plate 57, fig. 1, text-fig. 246). 



It is seen in Melonechinus muUiporus that there is a perfectly definite arrangement of 

 plates in the interambulacral areas and the same is true of all other members of the Palaeechini- 

 dae as I have shown. In this regard remarks made by JNIr. Agassiz require consideration. 

 Mr. Agassiz (1881, p. 78) says, "The verj^ peculiar splitting of the vertical rows of coronal 

 plates noticed by Quenstedt in Melonites seems to point to some structural peculiaritj' in the 

 Palaeechinidae such as I have described in the breaking up of a single interambulacral plate 

 in our recent Echinothuridae. It shows, ;it any rate, what some of the other genera of the 

 Palaeechinidae plainly show, that we find it impossible to define the number of rows [columns] 

 of coronal plates in the test just as we find it impracticable near the apical system of the regular 

 Echinids to ascertain how many rows of interambulacral plates there are present, as they appear 

 in that region of the test packed in as they best can find place and take up their regular and 

 symmetrical arrangement only later, while we maj' observe that in the Palaeechinidae this 

 symmetrical arrangement never takes place, the vertical rows of plates running in as best they 

 can, thus forming another important embryonic character of the Palaeechinidae." In regard 

 to this statement of Mr. Agassiz's, I have seen no evidence for the splitting of coronal plates in 

 Melonechinus or other members of the Palaeechinidae (p. 415). The arrangement of rows 

 [columns] of plates is perfectly definite in Melonechinus and other members of the Palaeechinidae 

 as well, as I have shown (p. 383). In fact they are perfectly definite in probably all Palaeozoic 

 Echini. I have examined a good many regular Echini living and fossil and never saw any 

 difficulty in ascertaining near the apical disc how many columns there are. An unsymmetrical 

 arrangement of plates such as Mr. Agassiz assumes is not a primitive character according to 

 the evidence that I know, and would not therefore show the " embryonic character of the Palae- 

 echinidae" even if it existed in that group, which it certain!}' does not. Mr. Agassiz (1881, 

 p. 81) himself says that Bothriocidaris is "by far the most primitive of all Echinoidea," yet 

 in that type (Plate 1, figs. 1, 2) there is surely no unsymmetrical arrangement of plates. 



The peristome is known in Melonechinus muUiporus only, in the f amilj^ of the Palaeechini- 

 dae, and here known only from a single specimen which is in the Princeton University Museum 

 Collection 1,464. As a whole, the specimen is neither striking nor especially good, and I found 

 it in exchange material. This unique structure is shown in Plate 56, fig. 7, as it occurs in the 

 specimen, and with some restoration to make clearer the relations of parts in Plate 56, fig. 8, 

 and text-fig. 48, p. 80. Orally in areas J and D there are two ambulacral plates somewhat 

 larger than other plates of the peristomal area. These plates I consider as the primordial 



