ORDOVICIAN BRACHIOPODA 85 



extended anteriorly and also posteriorly almost to unite with the cardinal process 

 from which it was separated by a low saddle representing a remnant of the median 

 elevation. 



These modifications leading to the definition of the septalium are unknown in 

 younger skenidiids and serve to distinguish Protoskenidioides from other members 

 of the family. In S. cf. costatus Cooper from the Gelli-grin Group, for example, 

 impressions of the smallest brachial valves known, which are just over 1 mm long, 

 show that the septalium was not bilobed but evenly convex anteriorly as in adult 

 valves, and that it contained a high cardinal process not yet differentiated into 

 a myophore and shaft but which was continuous with the median septum and 

 bore no sign of having been built up from the median edges of the brachiophore 

 bases. 



The likelihood that Protoskenidioides gave rise to Skenidioides by an accelerated 

 development of a well-defined septalium is strengthened by the close comparison 

 between the two genera in other characteristics such as the distinctive shape and 

 ornamentation. Even the inference that the spondylium in the pedicle valve of 

 Protoskenidioides is free does not militate against the relationship because a septal 

 support to the spondylium of Skenidioides cannot always be identified without 

 recourse to apical sections of the shell. Such a line of descent is also still consistent 

 with the suggestion (Kozlowski 1929 : 129) that the skenidiids were derived from the 

 finkelnburgiids : indeed the attitude of the slightly inclined, convergent brachio- 

 phore bases of Protoskenidioides is more reminiscent of Finkelnburgia than the later 

 skenidiids. 



Protoskenidioides revelata gen. et sp. nov. 

 (PI. 14, figs. 4-13 ; Text-fig. 8) 



Diagnosis. Subpyramidal, mucronate Protoskenidioides with a planar brachial 

 valve 47% as long as wide, lateral slopes ornamented by up to 18 costae with a broad 

 ventral median costa corresponding to a dorsal sulcus 0-4 mm wide, 1-5 mm antero- 

 median^ of the umbo. 



Description. Transversely semi-oval, mucronate Protoskenidioides with a sub- 

 pyramidal pedicle valve and a planar brachial valve averaging 47% (range 45% to 

 48%) as long as wide for 4 valves with a well-defined rounded median sulcus con- 

 sistently 0-4 mm wide, 1-5 mm antero-medially of the umbo of 4 valves; radial 

 ornamentation on lateral slopes consisting of up to 18 costae increasing in number 

 during growth by implantation with the right lateral consistently 0-2 mm wide in 

 4 valves, and the ventral median costa complementary to the dorsal sulcus about 

 0-6 mm wide in adult shells. 



Ventral interior with shallow spondylium, broadly convex anteriorly. 



In adult brachial valves the septalium, about half as long as wide, extended 

 anteriorly for an average of 23% (range 21% to 26%) the length of 5 valves but it 

 was not continuous with a long median septum which may have reached the anterior 



