256 Schuch< rt — Jackson on the Phytogeny of the Echini. 



Oculars and genitals. — " An ocular ])late in Echini overlies 

 an ambulacrum wholly and the two adjacent interamhulacra in 

 part on either side. Immediately on the ventral borders of 

 the oculars all coronal plates originate. It seems that at this 

 point the tissues exist which give rise to new plates. . 

 The live oculars are always present barring the excepted Ponr- 

 talesia. The genitals overlie the inter ambulacra in part, but 

 not the lateral borders of the same, and never reach the ambu- 

 lacra. In some cases the genitals may not reach the interam- 

 hulacra. Five genitals are typically present, but the posterior 

 genital may be wanting (spatangoids) or one absent as an aber- 

 rant variation. . . . 



" In the ancient Bothriocidaris the oculars are exceptionally 

 large, relatively to the size of the animal ; on the other hand, 

 the genitals are exceptionally small, relatively the smallest of 

 all known Echini. . . . 



" No pores have been observed in genital plates in Bothrio- 

 cidaris. It is possible thej 7 did not have genital pores, as such 

 are wanting in the young of Recent Echini ; more likely they 

 were present, but do not show in external view . . . Also 

 no pores have been observed in ocular plates of Bothrio- 

 cidaris. . . . 



" Genital and ocular plates are rare in Palaeozoic types, yet 

 excepting the Echinocystoida I am able to show them in all 

 families other than the Archaeocidaridae and in most genera. 

 After Bothriocidaris just considered, the leading character in 

 the Palaeozoic is for all the oculars to reach the periproct, and 

 to cover the ambulacra and in part the interamhulacra on either 

 side. Also the genitals reach the periproct, are larger than the 

 oculars, and cover the interamhulacra in part, but not wholly, 

 because the lateral borders of the interamhulacra abut against 

 the next adjacent ocular on either side." In Paleozoic Echini, 

 as a rule, " the ocular plates are imperforate. ... In 

 post-Palaeozoic Echini ocular plates have one pore not always 

 visible externally (Salenia, Arbacia) and very rarely a second 

 pore mav exist as a variant. I have seen only two or three 

 such" (86-89). 



Systematic value of oculars. — " A close study reveals char- 

 acters of importance to general morphology, to the evolution 

 of the group, to the relation of the species in the genus and 

 related genera, and to geographical distribution. Ocular plates 

 present an excellent systematic character which has been 

 largely overlooked. . . . 



"Early in my studies of these plates it was seen that they 

 had an important bearing, and observations were made on all 

 available specimens of regular Echini, Mesozoic and Recent. 

 In the fossils this is not always easy, as for purposes of study, 

 all five oculars and genitals must be observed, and they are 



