164 
PALAEONTOLOGY 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: 
(Spondylobolus) ? - Barroisella 
? 
Lingulepis 
Lingulops-Lingulasma- 
Dinobolus 
f Trimerella 
-{ Monomerella 
(_ Rhinobolus 
Returning to the genus Obolella, we find it also an important point of 
divergence. 
