96 ROBERT TRACY JACKSON ON ECHINI. 



164) and Diplocidaris desori (text-fig. 58). In both species the oculars are strongly exsert. 

 The specimen of Cidaris coronata (text-fig. 164, p. 149), which is in the Stuttgart Museum, is 

 exceptionally perfect for plates of the periproct, which are very rarely preserved in the 

 Mesozoic. The character of having all the oculars exsert is seen in the young of Recent 

 cidarids. It also occurs as the adult character of some Recent species and as an arrested 

 variant in species typically having some oculars insert as seen in the table, pp. 100, 101. 



Cidaris abyssicola (74 specimens) has typically all oculars strongly exsert, but in two 

 specimens all the oculars are narrowly insert. It is remarkable that the only variants in this 

 species should have this extreme character. 



Cidaris cidaris l is apparently a rare species. Ten specimens were studied, in nine of which 

 all oculars are strongly exsert. Four of these were kindly sent me by Dr. Mortensen. One 

 specimen has oculars I, V, IV insert as a progressive variant. 



Cidaris affinis is a difficult and interesting species presenting an exceptional amount of 

 variation. In the form from the West Indies and Florida in 161 specimens in the U. S. 

 National Museum, identified by Dr. Mortensen, the typical character by a small majority 

 is for all oculars to be exsert as in the young, and in the adults of Jurassic species. There is, 

 however, great variation, and only 30% have the typical species character of this locality. 

 As all exsert is the character, cases with more or less oculars insert may be considered as pro- 

 gressive variants. In 18 % ocular V only is insert, in 2 % oculars V, I and in 7 % oculars V, I, 

 IV are insert, which is the species character of Euddaris Iribuloides. In 0.6% oculars V, I, 



IV, II are insert, like typical progressive variants of Euddaris tribuloides; in 20% oculars I, 



V, IV, III are insert. This, which is a common character in this species, is uncommon in 

 tribuloides and rare in the Centrechinoida, where typically, if four plates are insert, it is the 

 bivium and posterior pair of the trivium. In 20% all the oculars are insert; this, which 

 is the extreme progressive variant for the southern form, is the typical character in the Medi- 

 terranean form of Cidaris affinis. Three specimens are aberrant with oculars V, IV, III insert. 

 These are sporadic variants and are apparently imperfect cases of I, V, IV, III. 



In the Mediterranean form of Cidaris affinis (frequently and incorrectly referred to Doro- 

 cidaris papillata = Cidaris cidaris) a quite different condition prevails. The ocular plates 

 are all insert (text-fig. 59) as the typical condition in 53 % of the 120 specimens seen. On 

 the proximal side (text-fig. 60), however, the oculars are all exsert as in the young, and in adults 

 of Jurassic species. This is of interest as showing a primitive character internally and a pro- 

 gressive externally in the same plates. It is due to the adoral beveling of periproctal plates 

 over the genitals so that they extend further and present a different relation to the oculars 



1 This is the Dorocitlaris papillala of authors. It is gen Tally agreed that this is the Echinus cidaris of Linnaeus, and the 

 type of Cidaris; also the three Recent species commonly attributed to Cidaris s. s. belong rather to Euddaris Punirl 

 (Bather; H. L. Clark, Ann. Mag. Nat. Hist., 1908-'09; Mortensen, 1909).. 



