PLATE 6. 

 Arbacia punctulata (Lamarck). 



Fig. 1. Woods Hole, Massachusetts. Diam. 30 mm. R. T. J. Coll., 6f>0. X 4. Almost completely tetramerous. 

 There are four teeth, ambulacra, interambulacra, and oculars, but five genitals. The absent parts are: ocular and 

 ambulacrum II, the right half of interambulacrum 1 and the left half of interambulacrum 2. From this structural 

 deficiency the right half of interambulacrum 2, which is related to ocular III, and the left half of interambulacrum 1, 

 which is related to ocular I. come together and are in contact throughout their length. Same specimen as photo- 

 graph, Plate 8, tig. 2. Page 39. 



Tripneustes esculentus (Leske). 



Fig. 2. Pernambuco, Brazil. Diam. 110 mm. R. T. J. Coll., 739. X 1.9. Completely hexamerous; six teeth, ambula- 

 cra, interambulacra, oculars, and genitals, but genitals 3 and 6 are fused. The added parts lie between oculars III 

 and IV. (Compare Plate 7, fig. 7.) Page 46. 



Fig. 3. The same specimen from the mid-zone showing an ambulaeral area of the usual width and an interambulacrum 

 narrower than is normal X about 0.9. Pages 46, 78. 



Fig. 4. Jamaica. Diam. 75 mm. R. T. J. Coll., S06. X 2.8. Partially hexamerous. Ocular and ambulacrum VI 

 lie between ambulacrum II and interambulacrum 2. Interambulacrum 6 consists of a single column of plates that 

 abuts against oculars II and VI (compare Bothrioeidaris Plate 1, tig. 2). Oculars I, V, IV are insert and genital 3 

 is split by a horizontal suture (compare text-fig. 190, p. 169). Page 48. 



Strongylocentrotus lividus (Lamarck). 



Fig. 5. Naples Station. Diam. 61 mm. R. T. J. Coll., 696. X 2.8. Highly distorted. The madreporite is widely 

 separated from oculars II and III, against which the two columns of interambulacrum 2 abut as usual; genital 5 

 is similarly separated from oculars V and I, the periproct reaches the interambulacral areas at several points. 



Pages 52, 63. 



Strongylocentrotus drobachiensis (O. F. M filler). 



Fig. 6. York Harbor, Maine. Diam. 42 mm. R. T. J. Coll., 7S1. X 3.8. Partially tetramerous specimen. Ambula- 

 crum IV fails to reach the apical disc, so that interambulacra 3 and 4 are in contact dorsally; there are five oculars 

 and genitals, ocular III and genital 5 are split by median sutures (p. 170); extra genital pores occur, two of which are 

 in interambulacrum 3. Page 44. 



Fig. 7. Massachusetts Bay. Diam 37 mm. R. T. J. Coll., 780. X 2.8. Partially tetramerous in that genital 4 is 

 wanting, oculars IV, V are in contact, interambulacrum 4 dorsally has in part a single column of plates, as in 

 Arbacia, Plate 4, fig. 11. Page 45. 



Fig. S. The same specimen seen from within. X 2.8. To show the partially single column of interambulacral plates in 

 area 1. Page 45. 



Fig. 9. Sullivan, Maine. Diam. 14 mm. R. T. J. Coll., 661. X 5. Completely tetramerous specimen; there are four 

 teeth, eight primordial ambulaeral plates, four ambulaeral and interambulacral areas, four oculars and genitals. 

 The absent parts are ocular and ambulacrum IV, the right half of interambulacrum 3, and the left half of inter- 

 ambulacrum 4. From this structural deficiency the light half of interambulacrum 4, which is related to ocular V, 

 and the left half of interambulacrum 3, which is related to ocular III, come together and are in contact throughout 

 their length. (Compare Plate 6, fig. 1, and Plate 7, fig. 2; text-fig. 218, p. 191.) Page 38. 



Figs. 10-12. Locality (?) Peabody Museum, Salem. X 5.6. 



Fig. 10. A normal spine to compare with Figs. 11, 12, which are abnormally inflated and rounded. A unique variant in 

 the species and family. Page 78. 



