ORDOVICIAN BRACHIOPODA 31 



growth of L. displosa is also unusual, although a similar acceleration in growth along 

 antero-lateral vectors was responsible for the subtriangular appearance of adult 

 shells of the Tremadocian L. lingulaeformis (Mickwitz) from the Baltic area (Gory- 

 anski 1969 : 38), the mid-Ordovician L. decorticata Cooper from the Elway Formation 

 of Virginia, and L. rideanensis Cooper from the Aylmer Formation of Ontario 

 (Cooper 1956 : 198, 203). All three species, however, are described as having an 

 ornamentation consisting solely of fine concentric lines. 



Lingulella collected from the Meadowtown and Rorrington Beds are indistinguish- 

 able in every respect (Tables 13, 14) and constitute L. displosa s.s. The small 

 sample derived from the Stapeley Shales, on the other hand, differs significantly in the 

 relative narrowness and very gentle biconvexity of the shell and in the trace of a 

 fine median septum in the brachial valve. These differences have prompted the 

 erection of a new taxon, but to indicate its essential affinity with the Meadowtown 

 and Rorrington forms this is at the subspecific level only. 



PALAEOGLOSSA Cockerell 1911 emended A. W. 



1848 Glossina Phillips : 370 ; non Wiedemann 1830. 



191 1 Palaeoglossa Cockerell : 96. 



1956 Palaeoglossa Cockerell ; Cooper : 194. 



1965 Palaeoglossa Cockerell ; Rowell in Williams et al. : H270. 



Diagnosis. Elongately oval, biconvex lingulellinids ornamented by fine dis- 

 continuous fila with the dorsal pseudointerarea not differentiated into propareas. 



Description. Elongately oval, biconvex lingulellinids with an acute curved 

 ventral beak ; surface ornamented by fine fila in addition to growth lines ; ventral 

 pseudointerarea slightly anacline divided by a narrow pedicle groove into striated 

 propareas with strong flexure lines, dorsal pseudointerarea orthocline, striated, 

 lenticular in outline and not divided into propareas ; low median ridge sporadically 

 developed in the brachial valve ; muscle impressions unknown except for traces of a 

 pair of umbonal muscle scars immediately anterior of the pseudointerareas of both 

 valves. 



Type species. Lingula attenuata J. de C. Sowerby in Murchison 1839. 



Type horizon. Meadowtown Beds. 



Discussion. Well-preserved valves from the Meadowtown Beds afford an 

 opportunity to learn more about the internal morphology of the topotypic Lingula 

 attenuata Sowerby which is the type species of Palaeoglossa Cockerell (now. subst. pro 

 Glossina Phillips 1848). The presence and degree of differentiation of the pseudo- 

 interareas indicate that the genus is best accommodated among the Lingulellinae. 

 Indeed in the absence of information about the principal muscle scars, Palaeoglossa 

 differs from Lingulella only in details. They include the greater biconvexity, the 

 more incurved beaks and the more elongately oval outline of Palaeoglossa, and 

 possibly the skeletal structure, because the Palaeoglossa shell tends to be more laminar 

 and its inner layers are not radially striated as in Lingulella. Differences between 

 Palaeoglossa and Pseudobolus Cooper (1956 : 194) as currently understood are even 



