, s s I :. 



to Mieve th.it tlif // have been derived from tlM pleim.tomai i.iu line of 



development. In other word- tli- /.'r//.r./i//" /.'/</><//'/<//'//. ri>-nrt,it<irii<l,t 



and //.-.- 'hou^'h distinguishable, are nevertheless so closely united, in one way 



HI aiiot her. that no Astern of classilirat ion ran allord to separate them very widely. 

 The most natural arrangement at present suggested is to let thorn follow that 

 Mitral type of the li'i-tri'jxi'ln. the I'litfllm-ea, ill the order named. Tin- arrange- 

 nitMit, however, must l>e regarded as only provisional, -im-e it does not take into 

 account the grand divisional line which ought to be drawn lietween the symmetric 

 anil asymmetry /.../,<. according to which the class may he divided into two 



great groups, witli tli> Nr<iy<//<iy./n, l'l> r<i/'<><l'i. < t^islhulirnndnnin, ]'<>li//itii<'n/,lin>-<i and 

 / on one side, and the I'rQtobmtckioto, lli'tcr<>/>i/<i and Ptt/MOMOfc] on the 



i. These two groups are di-tinct already in the oldest known fossiliferous 

 ,s so far as known then, we may say from the beginning. The members of 

 each are determined not so much by the presence or absence of strictly bilateral 

 symmetry of organi/ation as by their developmental history.* A tendency to 

 become twisted characterizes the whole class, but while it is a constant condition in 

 the second group, it is never as marked and often quite absent in the first 



As regards the relation of the IMlrrophontacca to the Vl< Hr<>fm<iriiil<i and 

 so-called /yyobritii' Inn in general, it is scarcely as intimate as usually supposed. 

 f.tr as known both groups are equally ancient, and comparisons between the earliest 

 types of each show that they are quite as distinct as any of the later ones. The 

 apertural slit and slit-band which the two groups possess in common, and which 

 i< the only important feature in which they agree, became established somewhat 

 earlier among the pleurotomarians than with the bellerophontids, from which it 

 follows that the former is probably the more ancient instead of the younger type of 

 the two. They may have had a common ancestor, but neither sprang from the 

 other. 



The derivation of the bellerophontids is -till <, Of all the known and 



sufficiently ancient .types only Sl,-nnihn-ii, whirl) we refer to the /V.//</<v,/. appears 

 atall likely to have been concerned in theii -evolution. In this genus the shell is 

 more or less strongly curved, the surface frequently cancellated, and the dors um 

 sometimes subangular. From such a type it is not very far to Cyrtolitfs. Hut 

 Cyrtolitfs, despite the fact that it is closely connected, by one species or another, to 

 more typical members of the suborder, is very different from Oirenrlla, which 

 includes the only Cambrian bellerophontid known. Nor has the genus yet been 

 found in rocks older than the Trenton. Although, in placing the /W//v./</*r 



omly 



u> ho. ronclo.iTi.ljr that th early or Paloocote rpnnUUT of the Qooogtoi. Ptmpota und 8ea|Aopod<i 



