298 ROBERT TRACY JACKSON ON ECHINI. 



species of the family. There are eleven columns of plates at the mid-zone in each area; a 

 similar number is seen in some areas of specimens of H. rarispinus, but a greater number exists 

 in H. pentagonus. The primordial interambulacral plates are in place in the basicoronal row 

 in each area, as in other species of the genus. In the second row there are two plates, in the 

 third row three, and in the fourth row four, these being the point of origin of their several col- 

 umns. The fifth column originates in the fifth row in areas 1, 5, and 4, but in the sixth row in 

 areas 2 and 3. Likewise the sixth column originates in the sixth row in areas 1, 5, and 4, but in 

 the eighth row in areas 2 and 3. It is to be noticed that a bilateral symmetry is expressed by the 

 rate of coming in of columns five and six; in both they originate a little earlier in the three 

 posterior interambulacra than they do in the anterior two. Too much weight should not be 

 attached to a single observation, but it is in direct accord with what I have found in the sym- 

 metrical bilateral development of other structures in various regular Echini. Columns 7 to 11 

 come in very soon after the sixth, either in succeeding rows, or very nearly so, and the eleventh 

 column is attained at or just above the ambitus in each area. Area 4 is imperfect and column 

 10 apparently originates far to the left of the center, as indicated in the restoration. Senescence 

 is strongly marked by the dropping out of columns of interambulacral plates in this specimen ; 

 the adambulacrals 1 and 2 in all areas excepting 4 drop out early, so that above that point 

 columns 3 and 4 assume an adradial position. In addition, in area 2, columns 7, 11, 10, and 9 

 drop out passing dorsally, so that near the apical disc there are only five columns remaining, 

 a structure which is a senescent reversion to a similar character seen as a stage in development 

 in its own youth, and doubtless to the adult condition of some adult ancestral stock from which 

 it was evolved. A similar dropping out of interambulacral columns more or less completely 

 occurs also in the other areas. I would not point to it other than as a representative of the 

 family, but it is noteworthy that Lepidocentrus rhenanus of the Devonian (Plate 20, fig. 7) has 

 a slow, not an accelerated development, and has five columns of interambulacral plates in the 

 adult. 



The peristome is small as in all of the family, and is evidently plated with ambulacral plates 

 only (as in rarispinus), although in this specimen they are only partially preserved (p. 287). 



The apical disc is imperfectly known (Plate 25, fig. 5), but something can be gathered from 

 the mold. The genitals are fairly clear and are high and wide. Spaces for the oculars exist, 

 although the impress of these plates is not marked. They are restored to fill spaces between 

 the genitals, thus making them all insert. The outline is preserved of a few small angular peri- 

 proctal plates, and others are restored in the figure by dotted lines. A partial mold of the lan- 

 tern orally shows that this structure existed, and was probably as in other species of the family 

 (Plate 27, figs. 4-6). 



I take pleasure in naming this species in memory of my close friend the late Professor 

 Charles Emerson Beecher, of Yale University, thus linking his name with Hyatt's in a species 

 that is one of the most striking of known Echini. 



