18 



cry nnd study has offered, not only that the forty-two species of 

 A(/(in'('orriiii(s were ooiifined to the uieoloyicnl ayes, from whioli they 

 have been descrilied. but that the genus and the entire family of 

 genera to which it >)elonged, so far as the biologic;d part or the phy- 

 siological functions of the animals are concerned, were annihilated 

 absolutely from the face of tlie earth, in the Subcarboniferous pe^ 

 riod of geological time. Tiny were nf)t annihilated at the same time 

 nor by any convulsion of nature. Instead of one spec-ies graduat- 

 ing into another, by improvement or decline, whicji may possibly 

 have been the case, in some instances of which we have no proof, the 

 general rule was that one species became extinct at one time and 

 place, and another became extinct at another time ;ind at another 

 place, and. in this way. not only tiie forty-two species whicli are now 

 known were obliterated, but all the uidvuown species belonging to 

 the genus, and all allied genera which belonged to the same family 

 were annihilated before the ("oal Measures or ( 'arboniferons period. 



ll.ATOCKINrs Sll.\i;()NENSlS. u. S[). 



Philc ], Fi<i- /. (r:i/(ioiiK fu'dc ricir; Fi<j. ^, ojifKisilc ricir: Fkj. H. 



IdlcniJ ricic. 



Species medium size, somewhat biturbinate. Caly.K funrel shaped. 

 ra])idly expanded at the arms, a little less than twice as wide as high. 

 No radial ridges, surface plane and smooth or. possibly, iinely gran- 

 ular. Ambulacral openings directed a little above a horizontal line, 

 and not visible iii a basal view. An ovarian pore on each side of the 

 pair of arms opposite to the azygous area, and they are all we have 

 detected in two finely preserved specimens. 



Basals form a disc about four times as wide as high. Tt bears a 

 slight band and has an hemisperical depression for the attachment of 

 the column. First primary radials large and wider than long, three 

 hexagounl, twoheptagonal. Secondand third primary radials together 

 smaller than the first. Second primary radials ipiadrangular. two or 

 three times as wide as long. Third ])rimary radials very little larger 

 lliaii the second, pentagonal, axillary, and in the ray opposite the azy- 

 gous area bears uijon each upper sloping side three secondary radials, 

 which gives to this ray two arms. Tn each of the lateral rays the 

 third primary radial bears upon each up])er sloping side two sec- 

 ondary radials. the last ones being axillary, and bearing upon each 

 ujipcr sloping side two tertiary radials. which gives to each of these 



