132 



ROBERT TRACY JACKSON ON ECHINI. 



last is a normal extreme progressive variant for the species, rare as a whole, and the only one 

 found at Dumpling Islands, yet taken on in this relatively youthful specimen. It emphasizes 

 the fact which I have found elsewhere that progressive characters are taken on early in life, 

 and are a matter of individual development and not of age. While the percentage of I only 

 insert is higher than the adult character for this locality, the progressive variant I, V, IV is 

 practically of the same percentage as in adults of the locality. The aberrant variants are 



El 



138 



139 



V 



139a 



TEXT-FIGS. 135-139a. Normal ocular plate arrangement in Sirongyloccntrotus drobachiensis (O. F. Mtiller). All 

 figures X 3. 



135. Frenchman's Bay, Maine. Diam. 33 mm. R. T. J. Coll., 805. All oculars exsert, a unique arrested variant for 

 this species; genitals 5 and 1 are split by secondary sutures. 



136. Frenchman's Bay, Maine. Diam. 40mm. R. T. J. Coll., 663. Ocular I only insert, a common arrested variant. 



137. Nahant, Massachusetts. Diam. 66 mm. R. T. J. Coll., 664. Oculars I, V insert, the typical character of the 

 species. 



138. Nahant, Massachusetts. Diam. 37 mm. R. T. J. Coll., 665. Oculars I, V, IV insert, a common progressive 

 variant. 



139. York Harbor, Maine. Diam. 28 mm. R. T. J. Coll., 678. Oculars I, V, IV, II insert, a rare progressive variant . 

 139a. Massachusetts Bay. Diam. 55 mm. R. T. J. Coll., C66. Oculars I, V, IV, II insert as in last figure, a rare 



progressive variant. 



