224 Mr. Williamson on the Distribution of 



Murchisoni, and Pinna folium. They are found at Robin Hood's Bay, and 

 near low water mark, under the heights of RockcHfF. At the former locality 

 the Hippopodium preponderates, and at the latter the two species of Pholado- 

 mya. The Pinna, which occurs at both places, is sometimes obtained of a 

 very large size, and one which I measured at RockclifF was two feet in length ; 

 but, as is usually the case with this fossil, it was so crushed that it could not be 

 preserved. 



Towards the lower part and about the middle of this division of the lias, are 

 ionnA Ammonites planicosta, with fragments of Pentacrinus Briareus and P. 

 vulgaris, (P. caput Medusce, Miller), principally broken columns, which are 

 very numerous, particularly at Robin Hood's Bay. Still higher in the series, 

 and towards the top of the sandy strata, I have procured Ammonites cornu- 

 copia, A. Turneri, and A. geometricus, especially opposite the Boulby Alum 

 Works; but I am rather undecided respecting these fossils, as they are rare. 

 Thestratanextin succession are softer, and separated by seams of irony nodules, 

 having below them at Robin Hood's Bay and Skinningrave, a bed which 

 contains the httle Plicatula spinosa. Towards the top of the lower lias rock, 

 Beiemnites compressus occurs, though not very plentifully. I found it princi- 

 pally at Boulby, where the highest beds are well exposed, 



2. Marlstone. — The next series of beds belongs to the division called the 

 Marlstone; and has an average thickness of about 130 feet. The first hun- 

 dred feet are composed of alternating sandy and shaly seams, and the next or 

 upper thirty of sandy shales, divided by bands of argillaceous, irony nodules, 

 which are sometimes united in solid strata, 6 inches thick. 



Towards the lower part of the division Cardium truncatum is very plentiful, 

 especially near Staithes. An interesting fact is connected with this shell. The 

 specimens which have both valves united, are found distinct from those with 

 the valves separated ; in the former the casts of the ligaments are preserved, 

 but in the latter they are wanting. 



In the sandstone seams towards the middle and lower part of the series, are 

 found the following fossils in great numbers and confusion. 



Ophiura Milleri, 0. loricata, Asterias Murchisoni, nob. in Mag. Nat. Hist., 

 Cidaris, with smooth spines, Dentalium giganteum, Beiemnites elongatus,Pla- 

 giostoma Iceviusculum, Modiola scalprum, M. Hillana, Avicula incequivalvis, 

 GrT/phcea Bullata ? Ostrea. 



Of these, Ophiura Milleri, 0. loricata, Asterias Murchisoni, Plagiostoma Iceviusculum, Modiola 

 scalprum, and the Cidaris are the most rare ; the others, especially the Ostrea and Avicula, form 

 immense masses, almost entirely composed of compressed and broken shells. 



This portion of the stratum is best examined on the shore between Staithes 



