52 ROBERT TRACY JACKSON ON ECHINI. 



The mineralogical structure of the sea-urchin plate, as seen in a section of a plate of Recent 

 Strongylocentroius drobachiensis, is crystalline calcite, with tlie axis of the crystal perpendicular 

 to the surface of the plate, as shown by extinguishment under the polariscope. The plate is 

 made up of a crystal which is optically continuous. Whether one, or more than one crystal 

 enters into the composition of a single plate is doubtful, l)ut it may jierhaps be compared to ice 

 on a pond, in which also the crystalline structure is optically continuous and the crystalline 

 axis is perpendicular to the surface. It may be noted that the same crystalline structure, with 

 axis perpendicular to the surface, was observed in the plates of Synapta, Asterias, and the 

 calyx plates of a young Antedon rosaccus. In the stem of Antedon, however, the crystalline 

 structure is parallel to the longer axis of the stem, not perpendicular to the surface. 



While in the sea-urchin the corona grows by increase in size of the plates, often to a relatively 

 great size, as in Cidaris, or the two plates in the second row of the posterior interambulacrum 

 of Micraster, it also grows by the addition of new plates. In the corona, the new plates, both 

 ^n the ambulacral and interambulacral areas, are always added dorsally, in immediate contact 

 with an ocular plate. This holds good for Palaeozoic as well as later types; the only excep- 

 tion appears to be the aberrant Pourtalesia jeffreysi Wyville Thomson, in which, according to 

 Loven (1883), in adults, at least, oculars are apparently wanting. In Bothriocidaris archaica 

 (Plate 1, fig. 2) it is seen that the ocular plates cover both the ambulacrum and interambu- 

 lacrum entirely, and in all other Echini, as already stated, the oculars overhang the ambulacrum 

 entirely and the interambulacra in part on either side, as in Melonechinus (Plate 50, fig. 6), and 

 Lovenechinus (Plate 41, fig. 3). It is true that in many, j^erhaps most Echini, the interam- 

 bulacral plates originate in contact with both the ocular and the genital plates in the angle 

 between them, but they apparently never fail to come in contact with the oculars and they 

 may not touch the genitals, as seen in Bothriocidaris archaica (Plate 1, fig. 2). In the posterior 

 interambulacrum of Ananchytes (text-fig. 175) and in adult Phormosoma placenta (text-fig. 170) 

 interambulacral plates originate against the oculars without touching a genital, and the same 

 may often be seen in abnormal specimens as in area 4 of Arbacia (Plate 4, figs. 11, 12), and in 

 the extraordinarily modified specimen of Strongylocentroius lividus (Plate G, fig. 5). It is strik- 

 ing in the Palaeozoic genera with many columns of plates that the new interambulacral plates 

 originate against the oculars, Lovenechinus, Plate 41, figs. 2, 3, as tliscussed more fully later. 

 What part, if any, the ocular plays in the origination of new ])lates is miknown, but at this 

 area they originate. Mr. A. Agassiz (1904, p. 80) notes that in sjiecimens of Lovenechinus 

 (Oligoporus) rnissouriensis and Lepidechinus iinhricatus the young interambulacral plates 

 originate against the ocular. The specimen of Lovenechinus to which he refers is that figured 

 in my Plate 42, fig. G, and the Lepidechinus in my Plate 03, fig. 7, as L. tessellatus sp. nov. 



The periproct grows by increase in the size of plates and the addition of new plates quite 

 independently of the corona. The peristome grows in area by increase in the size of basicoronal 



