LOVENECHINUS. 341 



we see that the younger plates in the placogenous zone close to the oculars are primaries, pass- 

 ing across the half-areas, and pore-pairs are uniserial. This simple condition of the young, 

 last added plates as a localized stage is directly comparable to the plates built in youth (Plate 

 42, fig. 1), or the plates throughout the whole area in the simpler genus Palaeechinus (Plate 

 31, fig. 1). Passing ventrally from the youngest plates to those somewhat older and struc- 

 turally more advanced, although there is some irregularity, we see that at the zone marked 

 X, X, in Plate 42, fig. 3, the plates for a short distance are alternately primaries, which are 

 expanded at the adradial suture, and occluded plates, which are cut off from the adradial 

 suture, but both meet the center of the area, and pore-pairs are biserial. This is like the 

 second stage in development seen ventrally, and like the character of Maccoya, the next 

 lower genus in which ambulacral plates are typically primary and occluded with pore-pairs 

 biserial (Plate 33, fig. 7). Thus, from ventral developing stages and from dorsal localized stages 

 in a single specimen can be read the relations of the type to associated genera in the family. 

 But this is not all that is shown by this wonderful specimen. At the point marked y, in the 

 left half-area (Plate 42, fig. 3), there are two plates, and at y in the right half-area there is one 

 plate, which lie in the middle of the half-areas and do not reach either the adradial suture or 

 the middle of the area. In other words, these are sporadic cases of isolated plates, and the 

 only ones seen in any specimen of the genus. As a character, they represent local progressive 

 variations toward just that addition of plates which is the character of Oligoporus, and is the 

 beginning of the system of isolated plates which is the main feature of Melonechinus. 



A young specimen in the Museum of Comparative Zoology Collection 3,140, which is only 

 32 mm. in diameter (Plate 39, figs. 1,2; Plate 40, figs. 1,2; Plate 41, fig. 2), is of much interest 

 as the youngest Palaeozoic echinoid seen. It is so nearly complete that with some restoration, 

 indicated by dotted lines, it is drawn spread out by the Loven method (Plate 40, figs. 1, 2). 

 There are two plates in the basicoronal row as restored, three in the second row, observed or 

 inferred, and four in the third row, as observed or inferred. The fifth column originates in the 

 fifth to the eighth row in four areas, but in area E this column is apparently represented by 

 only a single plate, which is in the thirteenth row. A sixth column is represented by a single 

 plate dorsally in the thirteenth row in area I. As it is a young specimen, there are fewer plates 

 than in an adult, there being only 13 or 14 plates in each adradial column in four of the areas, 

 but in area E, which is much narrower, there are 15 adradial plates on each side. This area 

 in the drawing of the internal mold is restored ventrally, but a good deal of this portion is shown 

 in Plate 40, fig. 2, drawn from a wax cast of the external mold, which therefore supplements 

 for this area the less complete drawing of the interior. In a large adult (Plate 41, fig. 1) there 

 are from 25 to 27 plates in each adradial column, about twice as many as in this young 

 specimen. (Compare studies of the interambulacrum, Jackson, 1899, p. 129.) 



In a dorsal view of this young specimen (Plate 41, fig. 2) oculars B, J, and probably H, 



