THE PENTAMEROITS SYSTEM AND VARIATION. 41 



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

 mordial ambulacral plates, four ambulacra and interambulacra in the corona, four oculars, 

 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, hut five interambulacra, oculars, genitals, and teeth. — In Arbacia 

 ■punctulata (Plate 7, fig. 1) an ambulacrum is entirely absent, but otherwise the specimen is 

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

 and oculars, ventrally five teeth and ten primordial ambulacral plates. There is no trace of 

 ambulacrum IV in the corona. Interambulacra 3 and 4 have each two columns 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 a,ssurance to either areas 

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

 corona to occupy the place that would have been filled by ambulacrum IV if it had developed. 

 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 the 

 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 genitals, 

 five teeth. — A very peculiar case is the Strongylocentrotus drobachiensis, 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 sub- 

 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 Ila 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 atlanticus, from Bermuda, 

 R. T. J. Coll., 895. It measures 62 mm. in diameter, and is not distorted. There 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 are five ambulacra, 

 IV and V being in contact throughout, as interambulacrum 4, also genital 4, are entirely 

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



9. Five ambulacra, four interambulacra, five oculars, four genitals, five teeth. — • A Toxo- 

 pneustes variegatus from Boca Ciega Bay, Florida (R. T. J. Coll., 820), 52 mm. in diameter, has 

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



