THE PKNTAMKKOrS SVSTKM AM) VARIATION 41 



Gallant, Patagonia, in the Museum of Comparative Zoology. It ha- live teeth and ten pri- 

 mordial ambulacral plates, four ambulacra and interainbula'Ta in tin- corona, four ocular-, 

 but five genitals. The wanting parts are ocular and ambulacrum V, the right half of inter- 

 ambulacrum 4 and the left half of interambulacrum 5. 



7. Four ambulacra, but five interambulacra, oculars, genitals, and teeth. In Arbacia 

 punctulata (Plate 7, fig. 1) an ambulacrum is entirely absent, but otherwise tin- -pecimen is 

 pentamerous. It is a small specimen, 22 mm. in diameter. Dm-sally there are five genitals 

 and oculars, ventrally five teeth and ten primordial ambulacral plates. Then' i- no trace i.f 

 ambulacrum IV in the corona. Interambulacra 3 and 4 have each two column- of plates 

 which extend dorsally to oculars III, IV, V, as usual in columns of these two areas. In addition 

 to the four columns, there are ventrally several plates representing a fifth column and one 

 plate a sixth column. These extra columns cannot be referred with assurance to either areas 

 3 or 4, but were in all probability developed below ocular IV and serve as space filler- in the 

 corona to occupy the place that would have been filled by ambulacrum IV if it had develo(>cd. 

 This specimen superficially is suggestively like Palaeozoic Echini where four or more columns 

 exist in an interambulacrum, the median columns being made up of plates which in general are 

 hexagonal, and the adambulacrals pentagonal. Here, however, the many columns are tin- 

 result of the union of two areas, not one area as in the Palaeozoic. 



8. Five ambulacra, four interambulacra, four oculars (one with two pores), four <jinittilx, 

 five teeth. A very peculiar case is the Strongylocentrotus drobachicnsis, Plate 7, fig. 5. The 

 specimen is good sized, 50 mm. in diameter and 30 mm. high. It has a pronounced bulging 

 on the median suture line of ambulacra I, II. There are four genitals and four oculars. One 

 of the oculars has two pores, indicating the termini of the radial water tubes of the two -ub- 

 jacent areas. There are five ambulacra, but interambulacrum 1 is entirely wanting. The 

 ambulacra I, II have each two columns of plates, but the 16 and Ha are very narrow columns. 

 It is worth noting that on the line of sutural contact of these two areas the outer pore of each 

 pore-pair lies above the inner one, the usual condition where an ambulacrum abuts on an 

 interambulacrum. There are five teeth and five pairs of auricles. 



A second specimen showing this same structure is a Toxopneustes atlanlicus. from Bermuda. 

 R. T. J. Coll., 895. It measures 62 mm. in diameter, and is not distorted. Then- are four 

 genitals and oculars, but one of the oculars has two pores, as in Plate 7, fig. 5, indicating that 

 the pores connected with the two associated ambulacral areas. There an- five ambulacra. 

 IV and V being in contact throughout, as interambulacrum 4, also genital 4. an- entirely 

 wanting. There are five teeth, but only eight gill cuts. 



9. Five ambulacra, four interambulacra, five oculars, four genitals, fiw 



pneustes variegatus from Boca Ciega Bay, Florida (R. T. J. Coll., 820), .V2 mm. in diameter. ha< 

 five ambulacra, but interambulacrum 5 and genital 5 are wanting. 



