Trilobites are well known fossils for two outstanding reasons:

1.  They were the first really dominant marine fauna to exist in the geological record (Cambrian Period).

2.  They are captivating to look at compared to many other invertebrate faunas.

During their long history they diversified into many different marine habitats and at the same time adapted their morphology to suite the conditions i.e. some became spiny, some blind, some very small and some exceedingly large. 

In local Carboniferous rocks they are thought to be common but in my experience they are difficult to find.  The photograph above curiously is taken in Salterforth Railway Cutting ( a disused railway line now a thriving ecological habitat with wild strawberries etc.).  The Pendleside Limestone and Pendleside Sandstone outcrop in the cutting.  The trilobite is exposed on the bedding plane of the Pendleside Sandstone.  Unfortunately the glabella (head) is very weathered and as such a tentative identification can made as Cummingella sp.