118 ROBERT TRACY JACKSON ON ECHINI. 



specimens, and the characters in regard to ocular plates present a wide range. When oculars 

 become insert, they do so in the sequence I, V, IV, II, III. Rarely V alone is insert, and when 

 this occurs, the character in so far is an approach to the Echinometridae (table, pp. 160, 161). 



Echinus as the type genus of the family is of especial interest. Of Echinus affir^is Morten- 

 sen, 605 specimens were observed in the National Museum. The material was from the 

 Albatross Station 2,094, off the Carolina coast in about 1,000 fathoms depth, and had been 

 identified by Dr. Mortensen. This species is remarkable as regards oculars in that all were 

 exsert without exception (text-fig. 115). This is the only species of Echini studied in which 

 with a large nimiber of observations no variation in this respect was seen. One marked pe- 

 culiarity of the species, which seems to have been overlooked, is the fact that, while the ambula- 

 cral plates up to the mid-zone or further consist of three elements each, dorsally the plates to 

 a greater or less extent may consist of two elements, as shown in text-fig. 115. The two-element 

 plates may be wanting (text-fig. 115b) or they may occur only near the upper end of the ambula- 

 crum, while in other specimens they may extend nearly or quite to the mid-zone (text-fig. 115a). 



In Echinus elegans and E. gracilis all oculars are exsert in the specimens seen. 



In Echinus esculentus (200 specimens) 97% have all oculars exsert (text-fig. 116), 1)ut in 

 3% ocular I is insert (text-fig. 117). This is the ocular that first comes in in this family 

 when one or more oculars are insert. The specimens with I insert, barring one exception, are 

 small or moderate sized, emphasizing the fact elsewhere noted that the incoming of oculars is a 

 matter of relative development, not of age and size of the individual. This species attains a 

 very large size, my largest from Plymouth, England, being 132 mm. in diameter. 



Echinus rniliaris (129 specimens) has 98% with oculars all exsert, but in one specimen 

 ocular V is insert, a character unusual for the family, and in two specimens I, V are insert, a 

 normal progressive variant for the species. Echinus microtuberculatus (299 specimens) has 

 typically, 96%, all oculars exsert; but in 2% ocular I is insert, in 1 %i ocular V, and in 0.3% 

 oculars I, V are insert as progressive variants. Two specimens are aberrant with oculars I, 

 IV insert. Of the species Echinus melo, only ten specimens were seen. Of these, 90 % have all 

 oculars exsert, but one specimen, 10%, has ocular I insert as a progressive variant. A speci- 

 men of Echinus melo from the Mediterranean, in the Peabody Museum, Salem, Massachusetts, 

 is of exceptional size, the largest sea-urchin I have seen. It measures 168 mm. in diameter 

 and 132 mm. in height, the circumference measuring 532 mm. The largest sea-urchin as far as 

 known is recorded by Messrs. A. Agassiz and Clark (1909), who say that the type of Sperosoma 

 giganteum A. Agassiz and Clai'k measures nearly 320 mm. in diameter. As this form is highly 

 flattened, its cubic contents is proportionately lessened. 



Of Echinus acutus (127 specimens), in 82 % all oculars are exsert, but in 18%) ocular I is 

 insert, showing that in this character it is much more progressive than the above species. In 

 a superb series sent me by the Plymouth (England) Biological Laboratory, there is great indi- 



