ORDOVICIAN BRACHIOPODA 77 



crests of the costae in the sulcus and gonocoels spreading antero-laterally along the 

 crests of the lateral costae. 



Dorsal interior with ridge-like cardinal process and wide notothyrium bounded by 

 secondarily thickened short brachiophores extending anteriorly for about one-fifth 

 the length of the brachial valve ; dorsal adductor impressions quadripartite, extend- 

 ing anteriorly for almost half the length of the brachial valve on either side of a low 

 median ridge with the subtriangular anterior pair the larger. 



Type material. 



length width (mm) 

 Holotype External and internal moulds of brachial valve 



(BB 35493a, b) 9-5 



Paratypes External and internal moulds of pedicle valve 



(BB 35494a, b) 9-0 117 



External and internal moulds of pedicle valve 



(BB 35495a, b) 10-5 15-0 



Horizon and localities. Whittery Shales : BB 35493 from Whittery Quarry 

 at the south end of Whittery Wood, near Chirbury (Grid Ref . SO 275981) ; BB 35494, 

 BB 35495 from exposures in the lower part of the west bank of the River Camlad, 

 60 yds north-east of Marrington Farm (Grid Ref. SO 272970). 



Discussion. The Platystrophia which occurs rarely in the Whittery Shales has a 

 highly distinctive concentric ornamentation of differentially developed lamellae. 

 Most Platystrophia are variably lamellose and some such as P. chama Eichwald (as 

 figured by Alichova 1953 : 131) strikingly so, but in P. caelata sp. nov. more than 

 4 mm long strong lamellae occur at regular intervals of about 0-5 mm and their 

 forward growth along the costae was accompanied by an equally regular sag of the 

 mantle edge. The resultant pattern is a series of subcircular, narrow canopies, 

 aligned along the crests of the costae, inclined at about 35 ° to the shell surface and 

 overhanging the posterior parts of alternating, depressed, subtriangular areas of 

 secondary shell. In the intercostate spaces, the lamellae lie flat on one another as a 

 series of overlapping tongues directed anteriorly. This ornament is characteristic 

 of all eight specimens of the genus collected from the Whittery Shales. It is un- 

 known in any other described Platystrophia, and in recognition of its uniqueness a 

 new species has been erected to include the Whittery shells. 



In other features, the new species appears to be unexceptional, although this may 

 be due to the meagreness of the sample. Judging from the clarity of gonadal im- 

 pressions and muscle scars which are deeply inserted even on the notothyrial floor 

 and about the cardinal process, specimens attaining a length of 10 mm were gerontic 

 individuals so that at least the Whittery representatives of the species were small in 

 size. All specimens showing the disposition of the costae on the fold and sulcus 

 were biplicate. The interiors of both valves show less secondary thickening than is 

 typical of later species of Platystrophia, but, even so, the brachiophore bases in the 

 brachial valve are heavily encased in secondary shell as are the ventral umbonal 

 chambers. 



