THE PENTAMEIIOUS SYSTEM AND VARIATION. 49 



confused by secondary sutures. There are six periproctal plates, probably a coincidence in 

 number only as six plates in that area may occur in a perfectly pentamerous specimen (text-fig. 

 204, p. 175). 



An Echinarachnius parma, from Chelsea Beach, Massachusetts, in the collection of the 

 Boston Society of Natural History, no. 348, is another case of a similar structure. This speci- 

 men (Plate 8, fig. 4; Plate 7, fig. 9) is 59 mm. in diameter. It is not unusual in appearance 

 except, for the added areas. There are five ambulacra and interambulacra ventrally and nearlj' 

 to the ambitus where the extra ambulacrum originates. The specimen is readily oriented by 

 the position of the periproct in 5 (Plate 8, fig. 4) and also by the proportionate size of the 

 primordial ambulacral plates (Plate 7, fig. 9), the la, Ila, III6, IVa, V6, being the larger plates 

 as usual, according to Loven's law. The sixth areas as seen are late in development and occur 

 between interambulacrum 1 and ambulacrum II. In the fused oculo-genital mass there are 

 six ocular pores, one to each area as usual. There are, however, only four genital pores. One 

 is wanting in 5 as usual, and also in 1, which is not usual. There is, however, a pore in the 

 aberrant interradial area 6. Ambulacrum VI originates just below the ambitus, has two 

 columns of plates and a petal as usual, and differs only in that plates below the petal are smaller, 

 and the petal is slightly narrower than in the other areas. Interambulacrum 6 originates 

 slightly later than ambulacrum VI and differs from the other interambulacra only in that the 

 plates are smaller. In this specimen the hexamerous variation was evidently not congenital, 

 for as shown, a pentamerous condition was maintained for a time, expressed in terms of growth 

 by the introduction of six horizontal rows or zones of ambulacral plates, as seen in area II 

 (Plate 7, fig. 9). Above this zone the added sixth ambulacrum is introduced, and it, with the 

 extra interambulacrum, is continued throughout succeeding growth to the apical disc. 



20. Pentamerous ventrally, six ambulacra, five interambulacra dorsally, six oculars, five 

 genitals. — This structure is shown in a specimen of Strongylocentrotus drobachiensis from York 

 Harbor, Maine (R. T. J. Coll., 847). The specimen is somewhat flattened, 27 mm. in diameter 

 and 13 mm. in height. There are five ambulacra and interambulacra ventrally, but within 

 5 mm. of the peristomal border a sixth ambulacrum appears and is extended to the apical disc. 

 The sixth ocular and ambulacriun lie between ocular and ambulacrum I and genital and inter- 

 ambulacrum 1. The relation is comparable to that shown in Plate 6, fig. 4, except that the 

 two ambulacral areas are in contact, as in Plate 7, fig. 5, since there is no intervening inter- 

 ambulacrum. The genital plates in this specimen are considerably split by secondary sutures 

 and somewhat distorted. 



De Loriol (1883, Plate 4, fig. 3a) describes a partially hexamerous Stomopneustes variolaris 

 that is referable to this type of variation. I studied the specimen in Geneva. Ventrally 

 there are five teeth, ten primordial ambulacral plates, five ambulacra, and five interambulacra. 

 Dorsally there are six oculars and a sixth ambulacrum, but there are only five interambulacra 

 and five genitals. 



