THE PENTAMEROUS SYSTKM AND VARIATION. 45 



several cases, each of the five ambulacra is involved, but IV appears to be more frequently 

 imperfect than any other area. 



16. Pentamerous ventrally, four ambulacra and five inter ambulacra dorsally, four oculars, 

 five genitals. This type is similar to that of 13 in that the ocular a oeiat. ! with an area ha* 

 disappeared, but differs in that there are five inter:imlml:irr:i. A Simrigylneeiitrotu- from 

 Calderwood Island, Maine (R. T. J. Coll. ,832), 4(1 nun. in diameter, is completely |>eni:imenui* 

 ventrally, but ocular II is wanting, and ambulacrum II falls short of the apical disc by 10 mm. 

 The two bordering interambu-acra, 1 and 2, are in contact dorsally. The dorsal plates of 

 column 1, in interambulacrum 2 and of column 2, in intcrainbulacrum 1 are large, not small 

 as is usual in dorsal plates, indicating that no new ones have been recently added, which i- in 

 accord with the absence of ocular II, from which the plates of these two columns take origin. 

 Three other specimens of the same species from Calderwood Island (R. T. J. Coll., 833-835) 

 and a Colobocenlrotus atratus from the Hawaiian Islands, in the State Museum at Albany, New 

 York, have a similar structure. In all, an ocular is absent, and the corresponding ambulacrum 

 falls short of the apical disc, but the associated interambulacra extend to the apical disc simi- 

 larly to the specimen just described. In the Colobocentrotus noted, ocular I is absent, and 

 ambulacrum I falls short of the apical disc. 



17. Pentamerous ventrally and throughout the corona, five oculars, but four genitals. 

 This line of departure from the pentamerous system is where only one genital is absent. It is 

 a typical condition in spatangoids where genital 5 is always absent, but occurs only as an aber- 

 rant variation in regular Echini. In Eutidaris tribuloides (text-fig. 185, p. 167) there are only four 

 genital plates, 1 being absent. Oculars I and II are in contact, and as a result interambulacrum 

 1 comes in contact with two oculars as it does in Bothriocidaris archaica and in the posterior 

 interambulacrum in spatangoids. This interambulacrum is also much narrowed dorsally 

 and at one point a single plate fills the area laterally. 



A more striking case is the Arbacia punctulata shown in Plate 4, fig. 11. In this remarkable 

 specimen there are five oculars but only four genitals. Genital 4 is absent so that oculars 

 IV and V are in contact. The interambulacrum 4 dorsally drops out to a single column of 

 plates; the single column and the ocular contact of the dorsal plate of the same as a 

 regressive variation is an exact repetition of the character seen in Bothriocidaris archaica 



(Plate 1, fig. 2). 



A quite comparable case is seen in Strongylocentrotus drobachiensis (Plate 6, figs. 7, 8 

 this specimen there are five oculars, of which I and II reach the periproct and V, IV, III are 

 exsert. There are only four genitals, number 4 being absent, so that oculars IV and V are 

 in contact. These plates are peculiar in that they are produced ventrally in an elongate fashion. 

 Interambulacrum 4 ventrally consists of two columns of plates as usual; above the mid-zone 

 it narrows, and the two columns give place to a single column of plates for three rows, that is. 



