Sf' 



PENTAMERUS LIMESTONE. 



117 



14. PENTAMERUS LIMESTONE. 

 (Part of No. 6. Pennsylvania Survey.) 



25. 



No. 1. Galeated pentamerus (P. galeatus). No. 3. Laeunose atrypa (A. lacunosa). 



2. Deep euomphalu^ (E. profundas). 4. Gcbhard's lepocrinites (L. gebhardi.) 



AU these fossils commenced in the distriet with this rock, and ended with it. 



This limestone takes its name from the Pentamerus galeatus, or Helmet pentamerus, for 

 which Galeated pentamerus is substituted on the authority of Noah Webster. This fossil is 

 well figured in No. 1 of the preceding wood-cut, which exhibits two views of it. It abounds 

 in this rock, and in the district is confined exclusively to it. In some localities this fossil is 

 in immense numbers, and is considered by Mr. Conrad to be the same as the one figured 

 under the name of Atrypa galeatus by Mr. Murchison, whence its specific name. None of 

 the English specimens have yet been received, and a doubt may be entertained as to iden- 

 tity. They make, however, a very near approach to each other externally ; and if not the 

 same, they may be considered as geographical substitutes, should their internal structure be 

 the same. There can be no doubt of the American fossil being a Pentamerus, as defined ; 

 but of the English one, it can not so positively be affirmed. The genus Pentamerus is one 

 which is readily recognized by its internal structure ; separating at the beak, for some dis- 

 tance, into five parts with smooth surfaces. 



No. 2 of the wood-cut is pecuhar to this rock, but it is rather a rare fossil. 



No. 3 is also somewhat rare, especially in the third district. It is important to give a good 

 figure of this fossil, as the name appears to be applied in Europe to a fossil like this, and to 

 the Atrypa wilsoni also of this rock. 



No. 4, shows two views of the lower part of the Gebhard lepocrinites : One in the perfect 

 state, wherein the internal structure is not evident ; the other shows the internal structure, 

 which consists of a series of circular disks or plates placed one upon the other : these are not 

 evident unless acted upon by the weather. It has a slight depression at the larger end, like 

 the one in the spine of an echinus, and by which it was attached to the upper part of the 

 singular fossil of which it forms a part. 



