ORDOVICIAN BRACHIOPODA 25 



and may be regarded as standard. These include length (sagittal), width (maxi- 

 mum) and depth (maximum) . Certain orientations for the measurement of internal 

 features are also coming into general use although they have not yet been standard- 

 ized. Those used in this paper are illustrated in Text-fig. 3, in relation to internal 

 moulds which lend themselves very well to estimating the forward extensions of the 

 bases of dental plates, brachiophores and sockets, or of muscle scars. 



V. SYSTEMATIC PALAEONTOLOGY 



Class INARTICULATA Huxley 1869 



Order LINGULIDA Waagen 1885 



Superfamily LINGULACEA Menke 1828 



Family OBOLIDAE King 1846 



Subfamily OBOLINAE King 1846 



OBOLUS Eichwald 1829 



Obolus subditivus sp. nov. 



(PI. 1, figs. 1-8) 



Diagnosis. Biconvex, subcircular Obolus almost as long as wide with a lenticular 

 ventral pseudointerarea extending anteriorly for 15% of the length of the pedicle 

 valve and a dorsal pseudointerarea not divided into propareas by an identifiable 

 median groove. 



Description. Subcircular, biconvex, Obolus with a mean length relative to 

 width of 95% (range 83% to 111%) and a mean depth relative to length of 14% 

 (range 12% to 17%) for 6 pedicle valves, and a brachial valve about one-tenth as 

 deep as long, both valves evenly convex transversely but flattened slightly anteriorly ; 

 beaks marginal ; shell surface ornamented by concentric growth lines and up to 10 

 impersistent low narrow plicae. 



Striated pseudointerarea of pedicle valve lenticular in outline, orthocline to ana- 

 cline in attitude, extending forward for 15% of the length of 6 pedicle valves (range 

 12% to 20%) and divided into 2 propareas by a shallow pedicle groove bearing a 

 fine low median ridge ; ventral muscle impressions unknown. 



Striated pseudointerarea of brachial valve obtusely triangular, orthocline, extend- 

 ing forward for only one-twentieth of the length of the brachial valve, median groove 

 not identifiable ; a pair of elongate muscle scars, possibly representing lateral muscle 

 bases, are vaguely impressed on either side of a low obscure median ridge. 



Type material. 



length width (mm) 

 Holotype External and internal moulds of pedicle valve 



(BB 35573a, b) 5-0 4-6 



Paratypes External and internal moulds of pedicle valve 



(BB 35574a, b) 3-3 3-i 



External and internal moulds of brachial valve 



(BB 35575a, b) 3*8 3-8 



