THE PENTAMEROUS SYSTEM AND VARIATION. 39 



from Mauritius, with complete or partially tetramerous tests, were found in 343 specimens 

 examined in the Museum of Comparative Zoology by Dr. H. L. Clark. Three of them fall 

 under the present head, with four teeth, eight primordial ambulacral plates, four ambulacral 

 and interambulacral areas in the corona, four oculars and genitals. In one of the specimens 

 two of the genitals are fused, and in one of them a genital is split into three parts. 



An Echinometm lucunter from Jamaica, in the Museum of Comparative Zoology, is com- 

 pletely four-rayed, with four teeth, eight primordial ambulacral plates, four ambulacra and 

 interambulacra in the corona, four oculars and genitals. The absent parts are ocular and 

 ambulacrum I and genital 1, the left half of interambulacrum 1 and the right half of 

 interambulacrum 5. 



5. Four ambulacra, interambulacra, and oculars, five genitals, four teeth. — This structure 

 differs from the last described only in that there are five instead of four genital plates. An 

 example is seen in Arbacia punctulata (Plate 8, fig. 2; Plate 6, fig. 1). The specimen is- small, 

 26 mm. in diameter, but is higher than usual proportionately, 19 mm. It is perfectly shaped, 

 not abnormal in appearance. There are five genitals, the only pentamerous part of the animal 

 observed, but there are only four genital pores, one being absent in the madreporite. The 

 specimen is oriented by the madreporite and planes of arrangement of the periproctal plates. 

 Only four oculars are present. The absent parts are ocular and ambulacrum II completely, 

 also the right half of interambulacrum 1 and the left half of interambulacrum 2. The corona 

 consists, therefore, of four ambulacra with three interambulacra and two half-interambulacra. 

 There are four teeth, four pairs of auricles, and eight primordial ambulacral plates. 



A second almost completely four-rayed Arbacia punctulata, from Woods Hole, Massachu- 

 setts, is in the American Museum of Natural History. This like the above is small, perfectly 

 formed, not at all distorted. It measures 25 mm. in diameter through the ambitus and 18 mm. 

 in height. There are five genital plates, but the madreporite has no genital pore, and lies in 

 close contact with genital 3 just as the madreporite lies in contact with genital 1 in Plate 6, fig. 1. 

 There are four oculars, ambulacra, interambulacra, and teeth throughout. The absent parts 

 are ocular and ambulacrum III, the right half of interambulacrum 2, and the left of interambula- 

 crum 3 (compare Plate 6, fig. 1). 



A third specimen of Arbacia punctulata, from Woods Hole, in the Museum of Comparative 

 Zoology, has a similar structure. It has four ambulacra, interambulacra, and oculars, but five 

 genitals, one depauperate and imperforate. The wanting parts are ocular and ambulacrum I, 

 the right half of interambulacrum 5, and the left half of interambulacrum 1. The teeth are 

 wanting. The Arbacia punctulata described by Osborn (1898) is similar to the above three, 

 the five genitals being the only pentamerous portion. In his specimen as gathered from the 

 description and figure, the absent parts are ocular and ambulacrum I, the right half of inter- 

 ambulacrum 5, and the left half of interambulacrum 1. 



