164 PALEONTOLOGY OF NEW' YORK. 



and the abrupt extinction of the entire group with the close of the Niagara- 

 Wenlock period constitutes one of the most striking features in their history. 



In Barroisella a considerable variation from Lingula is found in the char- 

 ter of the internal markings, accompanied by the development of the deltidial 

 callosities to such a degree as to indicate their specialization for purposes of 

 articulation, and it is here that we find one of the most striking of the few 

 instances observed among the brachiopods, of an evident tendency to span the 

 interval between the so-called inarticulates and articulates. Barroisella sub- 

 spatulata of the Genesee fauna affords the last phase in the development along 

 this line ; it had been preceded in Silurian faunas by the Lingula ? Lesueuri, a 

 form which has the articulating processes of Barroisella combined with the 

 septal characters of Dignomia or Glottidia. 



Future investigation may show that the little-known Spondylobolus is a 

 similar resultant which has come by the way of Obolella. The same tendency 

 to the development of articular processes in the pedicle-valve is also observable, 

 though to a less degree, in Trimerella and Monomerella, while the inception of 

 a cardinal process in the opposite valve has repeatedly manifested itself (See 

 note on page 1.) 



The immediate lines of derivation of, and departure from Lingula are ex- 

 pressed in the following diagram : 



(.S|ioiiclylol)oliis) ? - - BaiToisella 



{ Lingula f Lesxieuri) 

 Tomasiriii / 



Digiioinia Glottidia ^ 



> 



Ling-ulepis / Ling-iilops LiHf,'-ulasina fTrimerella 



^^^-^ ■{ Monomerella 

 Glossina -■ ,' r>u- i i 



Dinobohis LRhinobokis 



Returning to the genus Obolella, we find it also an important point of 

 divergence. 



