OCULAR AND GENITAL PLATES. 87 



reach the interambulacra. Five genitals are typically present, but the posterior genital may 

 be wanting (spatangoids) or one absent as an aberrant variation (Arbacia, Plate 4, figs. 11, 

 12; or Eucidaris, text-fig. 185, p. 167). 



The apical disc is large relatively to the diameter of the test in young Echini (Plate 2, 

 fig. 3) ; it is also relatively large in the adult primitive Bothriocidaris. With growth to the 

 adult the apical disc in Echini loses its preponderance but is still relatively large in the Cidaridae, 

 in Tiarechinus, in the Aspidodiadematidae and Saleniidae. In the Devonian and Carbon- 

 iferous Echini and in the Centrechinoida, other than those mentioned, the apical disc is rela- 

 tively small. In the Echinometridae which in other respects are amongst the most specialized 

 of regular Echini the apical disc is very small as it is also in the Exocycloida. As far as observed 

 commonly the apical disc in relation to the test as a whole grows with a progressively decreasing 

 ratio, a fact that can be verified from the figures shown here as text-figs. 63, 67, 128, 129a, 

 135, 137. Very large individuals do not necessarily have larger periprocts than smaller indi- 

 viduals of the same species (text-figs. 94-95, 151-153). In general a proportionately large 

 apical disc is a youthful and primitive character, a small one is a progressive character. 



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. There has been much confusion in regard to the genital plates of this 

 type, but, having enjoyed the privilege of studying perhaps the most perfect specimen known, 

 which is in the Berlin Museum, I have reached the following conclusions. 



In Bothriocidaris archaica (Plate 1, fig. 2) the oculars are very large; they are in contact 

 with the periproct dorsally, and meet in a continuous ring covering entirely both the ambulacral 

 and interambulacral areas. What I consider the genitals are five small plates lying in the dorsal 

 angles of the oculars and extending into the periproct. The relative position of the oculars 

 and genitals in this most ancient and, I believe, primitive echinoid is closely comparable to 

 that of the same plates in the larval Echinus (Plate 3, fig. 5). 



In Bothriocidaris pahleni (Plate 1, fig. 6) as shown by Schmidt (1874) the oculars are 

 also large; two of the genitals extend down between the oculars and come in contact with the 

 dorsal border of the interambulacra (one is absent), but two lie completely dorsal to the oculars 

 as do all the genitals in B. archaica. In this character, therefore, it is intermediate between 

 that species and the next. In Bothriocidaris globulus (Plate 1, fig. 9), as Schmidt (1874) showed, 

 the oculars are large, as in the two other species, but the genitals all extend down between 

 the oculars and reach the dorsal border of the interambulacra in their several areas as in all 

 later Echini. The position of genitals dorsal to the oculars is, of course, a common condition 

 in living Echini, the main peculiarities in Bothriocidaris being the relatively small size of the 

 genitals, and the fact that they may be excluded from contact with the interambulacra in B. 

 archaica, a unique specific character. 



