part 2 J SntlKGOTHYBIS AND SPIMTFE1U N.4 . 179 



, In this country the genus Syringotliyris first appears in the 

 Lower Oleistopora Zone (K x ), where it is represented by forms in 

 which all the essential characters of the genus are fully developed. 

 No two individuals are precisely alike; but the prevailing expression 

 is that of a small transverse shell, with a fiat cardinal area of which 

 the width is about twice the height. The transverse plate is not 

 deeply buried, is convex upwards, and the syrinx is relatively large. 

 This type of shell, exemplified by the form described hereafter as 

 Syringothyris principalis, sp. nov. [Syringothyris typa, auctt.], 

 prevails throughout the Oleistopora Zone, and is not found above 

 the lowest beds of the Zaphrentis Zone. 



Above the Oleistopora Zone occur two series of forms derived 

 from S. principalis, and here regarded as belonging to two species: 

 namely, S. elongata, sp. nov. and S. cuspidata (Martin). The 

 essential difference between them lies in the greater apical angle, 

 and tbe more depressed appearance of the former. 



The shells referred to S. elongata are less numerous, and exhibit 

 less variation than those of the other species, of which successive 

 mutations 1 are recognized. The further history of S. cuspidata is 

 as follows : — In r L x the prevailing t} r pe, S.. cuspidata mut. cyrto- 

 rhyncha \_S. teccta auctt.], is a shell distinctly larger than 

 S. principalis, and further differs from that form in that the 

 area is slightly concave, the brachial valve less transverse, and 

 the costse somewhat stouter. Although, in its typical form, 

 especially characteristic of the Lower Zaphrentis Subzone, forms 

 approaching S. cuspidata mut. cyrforhyncha occur in the under- 

 lying Oleistopora Zone. Owing to the lithological characters of 

 the rocks which frequently constitute the Zaphrentis Zone in the 

 South- Western Province, the sbells have undergone but little 

 recrystallization, and the minute textile ornamentation is frequently 

 well preserved. It is on this account that they have been compared 

 with the North American species S. texta (Hall). 3 



In the South- Western Province the shells found in the Upper 

 Zaphrentis and Oaninia Zones group themselves around S. cuspi- 

 data mut. exoleta \_S. carteri auctt.], which is most abundant in 

 and characteristic of horizon y and the Lower Oaninia Zone. 

 This form differs markedly from S. elongata, but is less sharply 

 differentiated from S. cuspidata mut. cyrlorhyncha. 



In the North and North-West of England, Syringothyris has 

 not been recorded from horizons lower than the Oaninia Zone; but 

 in rocks of that age it is represented by a form which in general 

 appearance, and in its stage of development, has close affinities 

 with S. cuspidata mut. exoleta, of which it is in all probability 

 merely a geographical variant. 



The Oaninia Zone in the central part of Ireland, and in Belgium, 

 is characterized by the occurrence of the remarkable Waulsortian 

 'knoll ' facies, and in those deposits Syringothyris is developed in 



1 In this paper ' mutation ' is used in the sense of Waag-en, and not in that 

 of De Vries. 



- Vaug-han. ' Burring-ton Paper ' [28] p. 364. 



