Vol. 52.] ASSOCIATED BEDS, IN N. DEVON AND W. SOMERSET. 269 



that ' I must now again remind the student that one of the chief 

 characteristics of Orihis rustica consists in its generally having a 

 small interpolated rib between each two of the longer ones, or 

 between those which extend directly from the extremity of the beaks 

 to the margin, the number of ribs varying according to the age of 

 the individual.' Among the specimens collected at Mullacott there 

 are examples of all sizes, and some quite as large as the largest found 

 in the Wenlock rocks of Shropshire. Its association here with 

 Stricklandinia lirata is highly interesting, and important in defining 

 the horizon of the beds ; with the exception of S. lirata and Lingula 

 mortensis, it is about the most plentiful fossil in these beds. 

 Pound at Mullacott and Barricane. 



MODIOLOPSIS BAEEICANENSIS, Sp. DOV. (PI. XL figS. 14 & 15.) 



This species approaches more nearly in shape Modiolopsis sub- 

 alatus, Hall, of the Niagara Group, than any British species. It is, 

 however, a much larger form than the American species, and the 

 umbo is situated nearer the anterior extremity. 



Length about 14 lines, greatest width about 7 lines. Sub- 

 rhomboidal in shape, posterior side greatly expanded. Anterior 

 extremity short and rounded. Umbo prominent. Surface marked 

 with moderately strong concentric lines, and near the anterior end 

 the concentric lines are crossed by numerous fine lines which extend 

 to the margin. 



The shell is still convex near the umbo, but it has evidently been 

 much flattened by pressure. 



Found in the cliffs on the northern side of Barricane, and Mulla- 

 cott quarry. 



Pterinjea mortensts, sp. nov. (PI. XL figs. 16, 17.) 



Several specimens have been found in the Mullacott quarry, 

 which exhibit characters sufficiently marked to indicate a new species 

 differing in several particulars from any other known British form. 

 The specimens are crushed and evidently somewhat distorted, but 

 show that the shell must have been broader than long, with a nearly 

 straight hinge-line. Anterior wing short and pointed, and separated 

 from the central part of the shell by a sulcus. Posterior wing long 

 and obtusely pointed. The surface of the shell is marked by 

 numerous radiating striae, which bifurcate as they approach the 

 margin. Some specimens show traces of concentric lines of growth. 

 The umbo is prominent, and situated about a third of the distance 

 from the anterior extremity. 



Width 16, length 10 lines. 



Pound, up to the present, only in the Mullacott quarry. 



Avicula, sp. (PI. XL fig. 18.) 



No perfect specimens have been found, but the fragment figured 

 shows that it is distinct from any British species. It, however, 

 somewhat closely resembles one of the species (Avicula undata) 

 from the Niagara rocks of America. 



Found in the cliffs, northern side of Barricane. 



