220 ROBERT TRACY JACKSON ON ECHINI. 



late, and in Metalia even the secondary tubercles are perforate. This perforate character is a 

 parallelism or a reversion to a feature typical of Palaeozoic species, the Cidaroida and Aulo- 

 donta, but is not known in other regular Echini excepting a few of the Saleniidae. Respiration 

 is apparently by ambulacral gills only. Sphaeridia are present. 



All the orders thus far considered have two columns of plates in each ambulacral area; 

 and one column (Bothriocidaroida) or two columns (Cidaroida, Centrechinoida, Exocycloida) 

 in each interambulacral area. In the latter the two columns succeed a single column repre- 

 sented by a single plate in the basicoronal row, seen in the young and also in the adult if not 

 resorbed. In succeeding orders we find two or more columns of ambulacral plates, and three 

 or more columns of interambulacral plates in each area respectively, and on this basis they 

 are considered a further remove from the primitive than any of the above types (p. 70). 



Of the Plesiocidaroida the type is Tiarechinus, which presents certain extraordinary 

 characters as shown by Lov^n (1883). The ambulacra in each area are composed of two 

 columns of low simple plates. The interambulacra in each area retain the primordial inter- 

 ambulacral plates in the basicoronal row, but in the second row there are three plates (text-fig. 

 31, p. 70). For a row of three plates to follow immediately the primordial interambulacral 

 plate of the first row is a character known in no other Echini. In Tiarechinus the three plates 

 of the second row are elongate, reaching the apical disc, so that each area consists of but two 

 rows of plates, a unique condition. The genitals are exceptionally large, nearly covering the 

 dorsal field of the urchin, and oculars are small and strongly exsert. With present knowledge 

 it is not closely affiliated with any other group. The three columns of plates in an inter- 

 ambulacral area, the retention of the primordial interambulacral plates, and the imperforate 

 and peculiarly distributed primary tubercles are all objections to including Tiarechinus in 

 the Cidaroida as done by Bather (1909a, p. 66). 



The Palaeozoic Echini included in the orders Echinocystoida and Perischoechinoida 

 structurally are more removed from the primitive as regards the ambulacrum and interambula- 

 crum than any Recent form. Further, there is no evidence in the development of modern 

 types that they were derived from these more complex forms. Following the facts there is 

 no alternative, as I see it, except to consider the Palaeozoic genera of the Echinocystoida and 

 Perischoechinoida as structurally differentiated on independent lines from later known forms 

 and on those lines further removed from the primitive than are the types now living. A some- 

 what comparable case is seen in Cephalopoda in which the extinct Devonian to Cretaceous 

 Ammonoidea are further removed structurally from the primitive Cephalopoda than are the 

 species of living Nautilus. 



Certain broad characters of these Palaeozoic orders may be considered first as they are 

 features of the group as a whole and can thus be disposed of and save repetition. Ambulacral 

 plates are always simple, with two pores each. There may be two columns of such plates in 



