150 SHELVE DISTRICT 



Discussion. Only a few incomplete moulds of a finely ribbed, obtusely geniculate 

 leptaenid have been recovered from the Hagley and Whittery Shales. There is 

 no doubt, however, about the generic identity of the specimens which compare 

 sufficiently closely with Kiaeromena cf. kjerulfi, occurring rarely in the Allt Ddu 

 Group of the Bala district (Williams 1963 : 464), to be regarded as conspecific. 



Order PENTAMERIDA Schuchert & Cooper 1931 



Suborder SYNTROPHIIDINA Ulrich & Cooper 1936 



Superfamily PORAMBONITACEA Davidson 1853 



Family SYNTROPHIIDAE Schuchert 1896 



Subfamily XENELASMATINAE Ulrich & Cooper 1936 



EUORTHISINA Havlicek 1950 



The genus Euorthisina Havlicek (1950 : 16) was not included by Biernat in her 

 review of the Porambonitacea (in Williams et at. 1965 : H523-536). But, as demon- 

 strated by Havlicek, the genus is undoubtedly a porambonitacean although unique 

 in its shape within the superfamily. This difference is related to the strong devel- 

 opment of wide interareas in both valves and the absence of a dorsal fold and ventral 

 sulcus. Such features are normally quite variable in other articulates but they are 

 so exceptional among the porambonitaceans as to prompt Biernat (1965 : H524) to 

 report that within the superfamily the interareas are 'commonly reduced and scarcely 

 visible' and that the 'anterior commissure is invariably uniplicate'. Neither 

 diagnosis is true of Euorthisina which is, accordingly, immediately distinguishable 

 from all other porambonitaceans. Through the kindness of Dr Havlicek, it is 

 possible to figure here internal moulds of Euorthisina moesta (Barrande) from the 

 Llanvirnian Sarka formation of Rokycany (PI. 28, figs. 4, 6, 7) which show the main 

 features of the genus (BB 37160-37162). 



Havlicek (1950 : 16) compared his genus with Xenelasma (Ulrich & Cooper 

 1936 : 631) which is also characterized by discrete dental plates and a septalium. It 

 is, however, possible that the internal similarity is an expression of convergence, 

 but until more is known about the ancestry of Euorthisina it seems more practicable 

 to follow Havlicek's lead and provisionally retain the genus within the Xenelasmat- 

 inae. 



Euorthisina cf. moesta minor Havlicek 

 (PL 28, figs. i-3, 5, 8) 



1950 Euorthisina moesta minor Havlidek : 87. 



Diagnosis. Subequally and evenly biconvex, transversely suboval Euorthisina 

 ornamented by fine lamellae and costellae numbering 3 per mm, 5 mm antero- 

 median^ of the ventral umbo ; parallel dental plates and septalium extending for- 

 ward for about one-fifth the length of their respective valves. 



Description. Transversely suboval Euorthisina with a gently convex brachial 

 valve about 60% as long as wide and a pedicle valve about 15% as deep as long with 



