46 ROBERT TRACY JACKSON OX ECHINI. 



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

 there 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 

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

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

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

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

 is 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 Ai-bacia punctulala from Florida (R. T. J. Coll., 889, Plate 4, 

 fig. 12). 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, in that interambulacrum 4 is completely developed, its two columns extending, as 

 usual, to the apical disc, but abutting against oculars IV and V without any contact with a 

 genital 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 

 diameter. It is 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 

 plates in 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 

 area. 



Hexamerous. 



18. Six ambulacra, interambulacra, oculars, genitals, and teeth. — In a fine large Tripneustes 

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

 a condition hitherto definitely known only in Strongylocentrotus Uvidus. The specimen 

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

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

 mens, p. 161), so that the specimen is oriented on the basis of the insert bivium and position 

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

 between interambulacrum 3 and ambulacrum IV. There are four normal genitals, but two, 

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

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

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

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

 29 mm., is the same that is found in the ambulacra of a normal five-rayed specimen measuring 



