ORDOVICIAN BRACHIOPODA 83 



Calcareous Ashes which has been compared with S. costatus Cooper from the Porter- 

 field of North America (Williams 1963 : 375). In internal proportions and the shape 

 of the shell the likeness is certainly close and, although the Shelve pedicle valve 

 tended to be relatively wider and deeper than the average Bala one, the differences 

 are not important. Indeed the only indication of a difference which may eventually 

 prove to be important is in the lack of development of a costellate ornamentation 

 on the Shelve Skenidioides. The sample is indistinguishable from S. costatus in the 

 number and wavelength of costae, but only the ventral median costa of the Shelve 

 specimens gave rise to costellae, whereas external branches arose freely from the 

 lateral costae of the Bala stock. This difference is not entirely related to the overall 

 greater size attained by the Bala shells. Within the size range of the Shelve speci- 

 mens, which are between 0-8 and 2-0 mm long, costellae branched from one or more 

 lateral costae in 7 out of 12 pedicle valves from the Gelli-grin Group. It is therefore 

 possible that the Shelve Skenidioides were characterized by a simplicity of ribbing 

 that persisted throughout growth, but larger collections will have to be obtained to 

 determine whether taxonomic recognition of the difference is worth while. 



PROTOSKENIDIOIDES gen. nov. 

 Name. One of the first skenidiids. 



Diagnosis. Transversely semi-oval, plano-convex shells with a high carinate 

 pedicle valve and sharply sulcate brachial valve ; radial ornamentation costate 

 with a wide ventral median costa forming the carina of the pedicle valve and cor- 

 responding to the dorsal sulcus ; ventral interarea long, planar, apsacline with an 

 open delthyrium, dorsal interarea shorter, planar, anacline ; shell probably im- 

 punctate. 



Ventral interior with pointed teeth and shallow spondylium apparently free of the 

 valve floor ; muscle and mantle canal impressions unknown. 



Dorsal interior with divergent brachiophores supported by flat-lying brachiophore 

 bases, discrete and transversely lobate in young shells but extending anteriorly as 

 elevated plates in adult shells and converging medially to form a ridge-like cardinal 

 process which is joined by a low swelling on the valve floor with a high long median 

 septum ; sockets narrow oblique, defined by concave elevated fulcral plates in 

 adult shells only ; adductor muscle field divided by median septum into two elongated 

 oval impressions bounded laterally by raised curved ridges ; dorsal mantle canal 

 system unknown. 



Type species. Protoskenidioides revelata sp. nov. from the Mytton Flags. 



Discussion. The skenidiid found in the Mytton Flags is represented exclusively 

 by fragile moulds of what must have been thin-shelled valves, but although the 

 smallest impression of the brachial valve recovered is about 4 times as long as that 

 of the protegulum, the size range (o-8 mm to 2-0 mm in valve length) is sufficiently 

 great to reveal some hitherto unsuspected aspects of the origin and development of 

 the skenidiid cardinalia. 



