180 DR. F. J. XOKTH OK [vol. lxxvi, 



an extraordinary manner. The shells vary within wide limits, but 

 all agree in attaining in the adult stage an unprecedented size, 

 accompanied in most cases by an almost complete abortion of the 

 eostas, especially of those on the pedicle-valve. The costse, when 

 present, are large, but poorly defined. The increase in the size of 

 the individual is not accompanied by any thickening of the shell- 

 substance. Usually, the area is flat, or more or less strongly 

 reclined ; while the median line along the floor of the sinus is 

 distinctly concave, and not convex as is the case in earlier forms, or 

 in contemporary forms, occurring in normal deposits. 



Were they not connected by intermediate forms, extreme types 

 might well be regarded as belonging to distinct species. The 

 variability of the shells found in the lower zones has already been 

 mentioned, and the great diversity of form in the deposits under 

 consideration is (in all probability) due to the accentuation of 

 different variable characters in different individuals. 



Much careful collecting will be necessary before these varied 

 forms can be further classified, and although distinctive names, if 

 available, would be very convenient, no useful purpose would be 

 served by the introduction of new names until the significance of 

 the variations is more fully understood. 



In the Seminula Zone Syrinqothyris is rare. It is represented 

 in the South-Western Province by forms resembling mut. exoleta, 

 and in that area does not occur above that horizon. It is not 

 known above C„ in the North- Western Province ; but in the 

 Midlands the genus persisted until Dibunophylluin times, and it is 

 there represented by S. cuspidata mut. cuspidata and S. elonyata 

 in D 2 . 



In the ' brachiopod-beds ' (Dibunophyllum Zone) of the Mid- 

 lands, and the ' reef-knolls ' of the Clitheroe and Craven districts, 

 Syrinyothyris is represented by forms which recall those found in 

 the abnormal rock-types of the Caninia Zone of Ireland. 



The holotype of Martin's S. cuspidata was obtained from beds 

 of D 2 age at Castleton, and closely similar shells are found in the 

 'brachiopod-beds' at Park Hill (Derbyshire), and at Axton (Flint- 

 shire). Principal T. F. Sibly has shown that the 'brachiopod-beds' 

 of Park Hill, Thorpe Cloud, and other localities in Derbyshire are 

 of Middle Dibunophyllum age (D 2 , subzone of Lonsdalia Jlori- 

 formis), and Dr. H. H. Bemrose's observations on the toadstones 

 of Derbyshire indicate that the shell-beds of Castleton are of 

 similar age. 



The age of the knolls at Cracoe and Clitheroe is less certain : 

 Vaughan in his last paper 1 expressed the opinion that they were, in 

 part at least, considerably older than the ' brachiopod-beds,' and 

 included representatives of the Seminula Zone, and perhaps of the 

 Caninia Zone also. 



It is not intended to discuss in detail the relative ages of the 

 Upper Avonian ' brachiopod-beds ' and ' reef-knolls ' ; but it . is 



1 Vaughan, • Dinant Paper ' [35] p. 15. 



