OCULAR AND GENITAL PLATES. 



Ill 



range of characters as regards oculars, from all exsert, the Jurassic and youthful character, to 

 all insert, and finally oculars and genitals separated by an interspace, a condition occurring 

 typically in no other regular Echini. The oculars reach the periproct in the sequence I, V, IV, 

 II, III, whereas in the two other suborders a different sequence occurs in two of the families, 

 the Arbaciidae and Echinometridae, in which the sequence is V, I and not I, V (see tables). 

 Considering the next suborder, the Stirodonta, we find in the Saleniidae the Mesozoic 

 species of Peltastes have all oculars exsert (127 specimens), as in Peltastes ivriyhti (Plate 4, fig. 7). 

 In Salenia scutigera (16 specimens) also all oculars are exsert, the same character being seen 

 in many other Cretaceous species of the genus. In the Cretaceous Salenia petalifera (20 speci- 

 mens) the oculars are all exsert in 95 %, but in one specimen, 5 %, ocular I is insert as a progres- 

 sive variant, in this feature being like the typical character of Recent species of the family. 

 Of many other species of Cretaceous Salenias seen, but not tabulated, all had the oculars exsert 

 excepting one specimen of Salenia clarki (?) Forbes, in the Jermyn St. Museum, in which ocular 

 I was insert. 



103 



105 



Text-figs. 102-106. — Ocular arrangement in the Saleniidae (pp. 174, 175). 



102. Salenocidaris profundi (Duncan). North of Tristan da Cunha, 1,425 fath. Diani. 10 mm. R. T. J. Coll., 

 667. X 4. Ocular I insert, the typical character; the suranal plate is dorsal to genital 3. 



103. AcrosalciiiaspinosaAgnssiz. Cornbrash, Chippenham, England. Diam. 14 mm. R.T.J. Coll. X 8. Oculars 

 I, V insert, the tj^jical character; the suranal plate is dorsal to ocular III. 



104. Acrosalenia hemicidaroides Wright. Forest Marble, near Malmsbury, England. Diam. 10 mm. R. T. J. Coll., 

 735. X 8. Oculars I, V insert, the typical character; the suranal plate is dorsal to genital 3. 



105. Acrosalenia willoiii Wright. Bradford Clay, near Cirencester, England. Diam. 12 mm. 11. T. J. Coll., 736. 

 X 8. All oculars insert, a progressive variant; the suranal plate is dorsal to ocular IIL 



The Recent species are very interesting in comparison. As Mr. Agassiz shows (1904, 

 Plate 21, fig. 2), in the young of Salenocidaris varispina, all the oculars are exsert as in adults 

 of Cretaceous species. Of adult Salenia pattersoni in 16 specimens, 69 % have ocular I insert 

 and 31% are arrested variants with all oculars exsert, like the character of the Cretaceous 



