220 MR F. R. COWPER REED ON 



Remarks on the Stratigraphical Distribution. 



It is noticeable that the greatest variety of species is met with in the Balclatchie 

 Group. In the underlying Stinchar Group only one species, represented by a single 

 individual, is so far known. The Whitehouse Group has yielded no more than two 

 species, while the Drummuck Group is particularly rich in members of the family. 



In the Silurian Beds it is remarkable that the Penkill Group contains a greater 

 abundance than the earlier Silurian horizons. 



This irregular development of specific types does not appear to be directly related 

 to the lithological characters of the strata, for though the argillaceous Balclatchie rocks 

 have the maximum of species, the sandy Drummuck Beds have many more types than 

 the fine-grained shaly Whitehouse Group. 



On the other hand, the arenaceous Mulloch Hill and Saugh Hill Groups are very 

 poor in representatives of the family, while the Penkill mudstones contain a compara- 

 tive abundance of forms. 



Other factors than the coarseness of the sediments must have been at work in 

 determining the relative abundance of species. It cannot be alleged that their absence 

 or rarity is due to their non -preservation in a coarse matrix, for the evidence of the 

 Drummuck Beds proves that sandy deposits lend themselves just as well as clays to 

 their conservation. Their independence of the rock-composition is as noteworthy in 

 Bohemia as in the Girvan district. 



In addition to the relative number of species on the various horizons, we must have 

 regard also to the relative number of individuals of each species, as evidenced by the 

 specimens in the collections. In this case, also, the maximum is found in the 

 Balclatchie and Drummuck Groups. On the former horizon, H. ardmillanensis, 

 H. girvanensis, and H. immemor are represented by so many specimens that they easily 

 outnumber all the other forms. In the Drummuck Beds the commonest species are 

 H. (Ortho.) ihraivensis and H. asteroideus, but the former is by far the most 

 abundant. 



With respect to the Silurian species, there are very few examples of any of them, 

 and less than a dozen have come under my notice from the Mulloch Hill and Saugh 

 Hill Groups. In the Penkill Group individuals are less rare, but pteropods do not 

 seem to be at all abundant members of the fauna. 



We are generally precluded from making any estimate, even with approximate 

 accuracy, of the actual relative abundance of species and individuals of any zoological 

 group of fossils in the beds deposited at a given period, the obstacles being the paucity 

 of material and the unsystematic method of collecting, or accidental manner by which 

 most museums have obtained their fossils. But in this case Mrs Gray's regular visits and 

 careful collecting from the sjpne district and horizons for many years past justify us in 

 considering the above conclusions as to the proportional distribution of the Hyolithidae 

 in the beds of the Girvan area to be a closer approximation to the truth than usual. 



