OCULAR AND GENITAL PLATES. 89 



as genitals. By a slip of the pen Mr. Agassiz in the paragraph quoted compares the five plates 

 in the dorsal angles of the oculars of Bothriocidaris to five anal plates in young cidarids. The 

 five plates in Bothriocidaris (Plate 1, fig. 2) are interradial in position and lie between the 

 oculars, whereas the five somewhat similar plates in young cidarids (text-fig. 61, Porocidaris 

 cobosi A. Agassiz, 1904, Plate 13, fig. 5), arc radial in position and lie between the genitals, for 

 the genitals alone reach the periproct, the oculars at this stage in cidarids being exsert. 



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 interambulacra on 

 either side. Also the genitals reach the periproct, are larger than the oculars, and cover the 

 interambulacra in part, but not wholly, because the lateral borders of the interambulacra abut 

 against the next adjacent ocular on either side. This character is shown in Palaeechinus 

 (Plate 31, fig. 4), Maccoya (Plate 34, fig. 6), Lovenechinus (Plate 42, fig. 6), Melonechinus 

 (Plate 56, fig. 6), Hyattechinus (Plate 25, fig. 5), Lepidechinus (Plate 63, fig. 7), and others. 



In one specimen of Lovenechinus missouriensis (Plate 41, fig. 2) in areas D and F, the 

 oculars are exsert, but in six other specimens of the species, with all oculars in place, all the 

 oculars reach the periproct. In Lovenechinus lacazei (text-fig. 243, which I owe to the kindness 

 of Dr. Bather), all the oculars are exsert. In Maccoya intermedia (Plate 33, fig. 11) one ocular 

 is exsert, four insert; and in Plate 33, fig. 12, all the oculars are exsert. In Lepidechinus tessella- 

 tus sp. nov. (Plate 63, figs. 7, 8) the oculars are all exsert as far as preserved, and the same 

 is true of Meekechinus elegans (Plate 76, fig. 5). These are the only cases of exsert oculars 

 observed in Palaeozoic Echini, and may be fairly considered as forerunners of the dominant 

 character seen in Mesozoic and many Recent regular Echini. 



In the Palaeozoic the oculars of Lepidesthes formosa (Plate 68, fig. 5) have two pores each. 

 Oculars in Lovenechinus missouriensis (Plate 42, fig. 6) show a single pore of small size close 

 to the ventral border of the plates, and the same is seen in Lepidechinus tQssellatus (Plate 63, 

 figs. 7, 8). These last two cases are views of the interior, not the exterior of the plates, and it 

 is possible that the pores did not reach the surface, or did not reach it so as to be seen in external 

 view, as is the usual case in Salenia. In all other Palaeozoic Echini seen, the ocular plates are 

 imperforate. Baily (1865b) figured two pores in Palaeechinus elegans. His original specimen 

 is shown in Plate 29, figs. 3, 6; Plate 31, fig. 4. The specimen does not have any pores in the 

 oculars and Baily's observation was an error, which has proved unfortunate as his figure of 

 the apical disc has been extensively copied. The characters of pores in the genitals and other 

 special features of these plates are considered in the next section. In post-Palaeozoic Echini 

 ocular plates have one pore not always visible externally (Salenia, Arbacia) and very rarely 

 a second pore may exist as a variant. I have seen only two or three such. 



