258 Sohuchi ii — Jackson on the Phylogeny of the Echini. 



described. "With few exceptions it was found that the larger 

 individuals in a species are typical as regards ocular plates, and 

 that variations, both arrested and progressive, are more 

 frequent in smaller individuals, often half grown as regards 

 size " (90-1). 



Genital pores. — "In very young Echini genital pores do not 

 exist. . . Typically, in post-Palaeozoic regular Echini there is 

 a single genital pore within the confines of each genital plate. 

 ... In the Ordovician Bothriocidaris genital pores are 

 unknown. ... In other Palaeozoic Echini genital plates 

 typically have more than one pore to a plate. There may be 

 two or three . . or there may be three to five in a plate. . . . 

 Instead of a few pores there may be numerous genital pores to 

 a plate, even as many as ten or eleven. ... It is possible that 

 in t} 7 pes where fine madreporic pores are unknown, some of the 

 larger pores served as madreporic openings. Otherwise all the 

 pores in genital plates doubtless connected with genital glands, 

 as in Recent Echini with accessory pores" (170-172). 



Secondary value of genitals in classification. — "As seen from 

 these studies, the genital plates have nothing to do with the 

 interambulacruin, which develops on either side of the oculars. 

 The genitals typically possess genital pores, and one of them 

 possesses madreporic pores, but both of these structures may 

 pierce other parts of the test. Genital plates may, therefore, 

 be considered as structures of secondary importance, of much 

 less morphological value than are the oculars " (173). 



The lantern. — " It is believed that the structure of the 

 lantern is of great value in systematic classification, and that 

 the structure of its several parts presents characters that are of 

 ordinal or subordinal value. As Dr. Mortensen pointed out 

 (1901), the structure of the teeth, keeled or unkeeled, is ' a very 

 important character, though it has hitherto received very little 

 attention.' Besides the teeth there are other features of value. 

 Briefly stated, the essential points are : teeth grooved or keeled ; 

 epiphyses narrow, or wide and united by suture ; the top of 

 the pyramids, as seen when the epiphyses are removed, a 

 smooth floor, or pitted ; foramen magnum deep, or shallow ; 

 angle of outline of the lantern depressed or erect ; compasses 

 present or absent" (177). 



Classification. 



The class Echinoidea Jackson defines as follows : 

 " The Echini, though possessing a wide range of structure, 

 may be described as animals possessing alimentary, reproduc- 

 tive, nerve, and water vascular systems within an enclosing 

 superficial pentanierous skeleton which bears movable spines. 

 There are from two to twenty columns of plates in each of the 



