ECHINID.E. 241 



well characterized by its remarkable ambulacra, abactinal system, and pedicel- 

 larise, can be traced. From Psammechinus, another line of development runs 

 out through Lytechinus to Toxopneustes and Tripneustes, characterized by 

 increasing specialization of the ambulacra, abactinal system, gill-cuts, and 

 pedicellariae. It is not easy to draw a line between Psammechinus and Lytechi- 

 nus, as the less specialized members of the latter genus have no noticeable gill- 

 cuts and only one ocular, or none, insert. For convenience, we have drawn an 

 arbitrary line based chiefly on the specialized globiferous pedicellaria? of Lyt- 

 echinus. It is probable that Gymnechinus, with its remarkable abactinal 

 system and globiferous pedicellarise, has been differentiated from Lytechinus 

 through such a group as Nudechinus, which is sharply distinguished from 

 Lytechinus by the thin, naked buccal membrane. From Psammechinus again, 

 still a third group of species has developed in the far south, for which Mortensen's 

 name Parechinus is employed. Of this group, angulosus is nearest the original 

 stock in its ambulacra, spines, and pedicellarise, but shows decided specialization 

 in the abactinal system and buccal membrane; annulatus is a very close ally. 

 In magellanicus, the buccal membrane shows further loss of calcareous matter 

 and the globiferous pedicellarise, though variable, are more specialized than in 

 angulosus. The most extreme member of this group is huttoni, as shown by the 

 globiferous pedicellarise and the tuberculation of the test. 



While it is impossible to show in a linear arrangement, the relationships of 

 the genera as here pointed out, the following table will indicate the limits which 

 are assigned to each one. It will be observed that it is not possible to 

 use the generic names Sterechinus Koehler, Pseudechinus Mortensen, Proto- 

 centrotus Doderlein, Notechinus Doderlein, or Selenechinus de Meijere. It is 

 to be regretted that no one of them is available for the little group of species for 

 which the name Nudechinus is herein proposed. 



Periproct approximately central; if two oculars reach it, they are typically I and V. 

 Buccal membrane more or less heavily plated (except L. rufus) ; a primary tubercle 



on every ambulacra! plate. 



Oculars plates all exsert; abactinal system well covered with tubercles; gill- 

 cuts insignificant; valves of globiferous pcdicellarise with lateral teeth . . Psammechinus. 

 Ocular I often, and in some species ocular V also, insert or nearly so; abactinal 

 system usually with few tubercles; gill-cuts, in specimens over 30 mm. h. d., 

 usually deep and sharply defined; valves of globiferous pedicellariae with- 

 out lateral teeth Lytechinus. 



Buccal membrane not plated but with more or less numerous, small, scattered 



plates or none in addition to primordial ambulacrals. 

 Gill-cuts shallow and not sharply defined. 



Poriferous areas not very broad; pore-pairs in arcs of three. 



