78 ROBERT TRACY JACKSON ON ECHINI. 



maries on adambulacral columns and exist alone on other interambulacral columns and on 

 the ambulacrum (Pholidocidaris, Plate 73, fig. 3; Perischocidaris, Baily, 1874, Plate 4). 

 Secondary spines may exist without any primaries. Such is the condition in all the Palae- 

 echinidae (Plate 52, figs. 10, 11), in Lepidesthes (Plate 70, fig. 4), and, judging from the small 

 and uniform size of the tubercles, in Lepidechinus (Plate 63, fig. 3) such spines and tubercles 

 may be fairly compared in size to those characteristic of Clypeaster and the usual type seen 

 in clypeastroids. Such small spines as occur in Bothriocidaris (Plate 1, fig. 3) may probably 

 be considered as primaries on account of the perforate tubercles and the small size of the whole 

 animal. As far as known, spines are very uniform in character within the species in the 

 Palaeozoic, cases of marked deviation, such as occur in some Cidaridae, being almost unknown 

 in these older types. From this uniformity in the Palaeozoic, spines present very good species 

 characters, which is fortunate, as they so often occur in a dissociated condition among fossils. 



As discussed (p. 51), spines are typically internal, and the evidence indicates that they 

 were internal, that is, covered by living epithelial tissue, in all Palaeozoic types excepting 

 Miocidaris and possibly Archaeocidaris. 



Stout, thick, and club-shaped spines are largely characteristic of the Cidaridae and a few 

 other types. A specimen of Strongylocentrotus drobachiensis in the Peabody Museum at Salem 

 is interesting as showing thick club-shaped spines (Plate 6, figs. 11, 12), which are quite cidarid 

 in appearance. These spines are numerous but rather local in distribution, not covering the 

 whole specimen. The specimen is of normal size and appearance otherwise, and it appears to 

 be merely a rare variation. This was the only such case seen in 33,000 specimens of this 

 species examined, nor was such a structure seen in any other species of the family. 



Tubercles as well as the spines with which they are associated may be divided into primary, 

 secondary, and miliary. Primary tubercles may be perforate or imperforate. They are 

 perforate in all Palaeozoic species where they occur; they are also perforate in the Cidaridae, 

 Centrechinidae, and some of the Saleniidae, also in some spatangoids. Primary tubercles 

 are imperforate in the remaining Centrechinoida, the Holectypina, and many of the Spatangina. 

 Secondary tubercles are imperforate in the Palaeozoic, whether they occur with primaries or 

 alone, as in the Palaeechinidae. They are also imperforate in the Cidaroida and in Acrosalenia, 

 but they are perforate in the Centrechinidae and certain clypeastroids and spatangoids 

 (Clypeaster, Metalia, Meoma). Perforation of tubercles is evidently a character of primitive 

 types and of some specialized types of clypeastroids and spatangoids. 



Primary tubercles in a species occur with a definite distribution which is characteristic, 

 as seen especially well in the Centrechinoida, a given number typically existing on a plate. 

 It seems that the distance between tubercles which is maintained in a species is the real feature 

 of their distribution. If, by chance, a plate is narrower than usual, as in Tripneustes (Plate 

 6, fig. 3), the tubercles are their usual distance apart, but there are fewer to a given plate; 



