GEOLOGICAL PALAEONTOLOGY 85 



the Wealden. Perhaps the swarms of Beyrichia that 

 characterize the uppermost parts of the Silurian may 

 indicate the oncoming of Old Red Sandstone lake- 

 conditions ; but closely allied forms abound in the 

 Upper Ordovician, where no such prospect seems 

 probable. In any case, detailed and special knowledge 

 of these small " bivalves " is required if marine forms 

 (which are common in the Chalk) are to be distinguished 

 from those of fresh water. In like manner, it would be 

 difficult, from study of the carapace alone, to ascribe to 

 a crayfish fluviatile, and to a lobster marine, habits of 

 life. 



Perhaps the most satisfactory palaeontological indica- 

 tions of either terrestrial or fresh-water origin for a 

 deposit are of a negative type. If fossils occur in a 

 stratum, and none of them belongs to exclusively marine 

 groups, the prevalence of one or the other of the non- 

 marine conditions may be presumed. While most 

 fluviatile Invertebrates have near allies living in the sea, 

 many phyla are absolutely restricted to the latter region. 

 Again, while it is possible, and by no means unusual, for 

 inland organisms to be drifted into the sea, the reverse 

 process is so unlikely as to be practically negligible. 



A very instructive case of faunal modification due to 

 physiographical change occurs in the Upper Silurian 

 rocks of Shropshire ; a similar phenomenon (at almost 

 the same horizon) has, recently been recognized in 

 America. In the grey flags of the Upper Ludlow series, 

 Protrematous Brachiopods, such as Orthis lunata and 

 Chonetes striatella, are present in extraordinary pro- 

 fusion (PI. vi. fig. i). They are accompanied by a sub- 

 ordinate Molluscan fauna, in which species of Pterinea, 

 Modiolopsis and Orthoccras are the most abundant types. 

 As the flags pass up towards the Downtonian sandstones, 

 the proportions of the groups undergo reversal. Almost 



