203 
1916-17.] The Hurlet Sequence and the Abden Fauna. 
shows that though the different calcareous horizons can be traced over the 
whole of Central Scotland, yet they vary considerably both in thickness 
and in character when traced from district to district. This feature is 
especially noticeable in the case of the Hurlet and Main Hosie Limestones, 
the former in the Renfrew and North Ayrshire districts, and the latter in 
the Bathgate Hills. The terrigenous sediments, though less constant in 
character than the limestones, can nevertheless be followed for considerable 
distances in which they retain well-marked lithological features, so that 
groups of sediments occurring between two known limestone horizons can 
often be identified over wide areas. 
XIII. Paleontological Comparisons and Considerations. 
It would be impossible within the limits of this paper to give a detailed 
analyses of the fossil contents of each of the limestone horizons under 
consideration, so that only the broader palaeontological features will be 
indicated. 
The Blackbyre or Coral Reef Limestone is characterised by the great 
abundance of colonial and solitary corals and brachiopods which it contains. 
The colonial corals belong mostly to the genus Lithostrotion , and include 
L. irregulare, L. junceum, and L. portlocki. They occur in reef -like 
masses over a large part of North Ayrshire, at Corrieburn, St Monans, 
Aberlady Bay, and Dunbar. The White Coral Limestone at St Monans has 
a thickness of about 18 feet. The corals evidently occupy the position 
in which they originally grew, though they are now bent and distorted 
by the pressure of the superincumbent strata, a not uncommon feature 
in existing coral reefs. 
The solitary corals may be distributed throughout the limestone in 
single individuals, or they may occur in regular bands from 1 to 2 feet 
in thickness. The bands are partly calcareous and partly argillaceous, 
the corals being most abundant in the latter, from which at certain localities 
they can be gathered in great numbers. The genera represented include 
Aulophyllum, Cyatliopliyllum , Cladochonus, Clisiophyllum, Dibunophyl- 
lum, Koninckophyllum , Zaplirentis, and others. 
The Blackbyre Limestone is also characterised by its extreme richness, 
both in species and individuals of brachiopods, thick bands being made 
up of the crushed shells of a single species such as Productus semi- 
reticulatus or P. longispinus. Amongst the genera represented are 
Dielasma, Spirifera, Martinia, Peticulccria, Spiriferina , Athyris, Retzia, 
Rhynchonella , Strophomena, Streptorhynchus, Schizophoria, Ghonetes, 
and Productus. 
