ROBERT TRACY JACKSON ON ECHINI. 



three plates (Plate 6, fig. 8). Above this point two columns are built again for a short distance 



being at this point three plates in column 1, and two in column 2. Proceeding dorsally' 



a change again occurs, and two single plates are built; the last is very high and narrow unlike 



anythmg seen in any other echinoid. This structure is very difficult to see on the exterior 



the test, but on the interior (Plate 6, fig. 8) it is perfectly plain. The dropping out to a 



column is comparable to the typical character of Bothriocidaris and to the peculiar 



Arbacm (Plate 4, fig. 11). Dorsally, interambulacrum 4 doe, not reach the ocular plates but 



separated for a short distance in which ambulacra IV and V come in contact and alone 



reach oculars IV and V. 



Another case was found in an Arbacia punctulata from Florida (R. T. J. Coll 889 Plate 4 

 The specimen is 33 mm. in diameter. It is pentamerous throughout except for the 

 absence of genital 4, which is wanting, as in Plate 4, fig. 11. It differs from that specimen 

 however, m that interambulacrum 4 is completely developed, its two columns extending as 

 al, to the a pl cal disc, but abutting against oculars IV and V without any contact with a 

 gemtal as that plate is absent. Again, a similar structure is shown in a specimen of Toxo- 

 pneustes atlanticus from Bermuda (R. T. J. Coll., 898). This specimen measures 53 mm in 

 s quite pentamerous throughout except that genital 4 is wanting. Oculars 

 IV and V are therefore in contact and cover completely ambulacra IV and V and interambula- 

 crum 4 as in the Arbacia, Plate 4, fig. 12. In this Toxopneustes there are two columns of 

 m contact with the two oculars, but the plates are small. These several cases demon- 

 strate that the absence of a genital does not cause a loss of the corresponding interambulacral 



fl.rpfl 



area. 



Hexamerous. 



18. Sv ambulacra, inter ambulacra, oculars, genitals, and teeth.- In a fine large Tripneustes 

 escukntus from Pernambuco, Brazil (Plate 6, figs. 2, 3) there are six areas throughout 

 L cond,tion hitherto definitely known only in Strongylocenlrotus lividus. The specimen 

 measures 110 mm. in all diameters as it is perfectly circular in outline. It is 58 mm in height 

 this species, when two oculars are insert, they are the bivium (see tabulation of 703 speci- 

 1), so that the specimen is oriented on the basis of the insert bivium and position 

 the madreporite. Obviously the additional ambulacrum, ocular, and interambulacrum lie 

 .tween mterambulacrum 3 and ambulacrum IV. There are four normal genitals, but two 

 3 and 6, are fused into a single plate, a somewhat rare condition, but similar cases are 

 en m Strongylocentrotus and Tripneustes (text-figs. 195, 196, p. 169). There are six oculars 

 lying between the tips of genitals 3, 6. The ambulacra are all normal in appear- 

 3 and at the mid-zone measure 28 mm. in width, except V, which is 29 mm. The width 

 the same that is found in the ambulacra of a normal five-rayed specimen measuring 



