40 ROBERT TRACY JACKSON ON ECHINI. 



In Yale University Museum a specimen of Arbacia lixula from the Mediterranean has 

 four areas. It is one of a lot of four specimens (cat. no. 199). There are five genital plates, 

 but only four oculars and four ambulacra and interambulacra which extend from the basi- 

 coronal row to the apical disc. The lantern is wanting. The absent parts are ocular and 

 ambulacrum I, the right half of interambulacrum 5, and the left half of interambulacrum 1; 

 therefore the left half of interambulacrum 5 and the right half of interambulacrum 1 are in 

 contact and form a single area (compare Plate 6, fig. 1). This area is wider than usual for an 

 interambulacrum, and the plates bear more primary tubercles than usual, but the tubercles 

 are spaced about as in other areas, and in normal specimens. This indicates, as shown in the 

 reverse condition in Tripneustes (p. 47), that tubercles and spines are distributed at a given 

 distance apart, rather than a given number to a plate, as a species character. Genital 1 is 

 somewhat distorted. Professor Verrill (1909) has given an excellent photographic figure of 

 this specimen. 



A small specimen of Strongyloccntrotus drobachiensis (Plate 7, fig. 3) has much the same 

 character as the above described Arbacias. The specimen is normally shaped except that it 

 is unusually high, dome-shaped. It is 35 mm. in diameter and 24 mm. high. There are four 

 ordinary genitals and a small plate between the madreporite and ocular I may be considered a 

 depauperate and imperforate fifth genital. There are four oculars. The specimen is oriented 

 by the madreporite and the two insert ocular plates which in this species mark the bivium. 

 The absent ocular on the basis of this orientation is evidently that one which should lie l^etween 

 the madreporite and the depauperate genital next to it. The absent part of the corona is 

 ambulacrum II, also the right half of interambulacrum 1 and the left half of interambulacrum 

 2. The lantern is wanting, but there are four pairs of auricles indicating a four-parted lan- 

 tern. A similar structure occurs in a specimen of the same species from Dumpling Islands 

 (R. T. J. Coll., 818). It is small, 15 mm. in diameter, not distorted, and completely four-rayed 

 except for the five genitals. The madreporite has no genital pore. The missing parts are 

 ocular and ambulacrum II, the right half of interambulacrum 1 , and the left half of interambula- 

 crum 2. 



6. Four ambulacra {bui ten primordial ambulacral plates), four interambulacra and oculars, 

 but five genitals and teeth. — This combination was found in a specimen of Strongylocentrotus 

 drobachiensis from Dumpling Islands, 5.5 mm. diameter (R. T. J. Coll., 819). Being very 

 young, all the genitals are imperforate except for the madreporic pores. Ocular I only is insert. 

 The missing parts are ambulacrum and ocular II, the right half of interambulacrum 1, and 

 the left half of interambulacrum 2. Surrounding the five teeth are ten primordial ambulacral 

 plates. This indicates that it began life as a pentamerous individual, but no trace of a fifth 

 ambulacrum is seen in the corona. 



A second specimen with these characters occurs in an Echinus magellanicus, from Port 



