ORDOVICIAN BRACHIOPODA 53 



(Grid Ref. SJ 380018) ; BB 35503, 35504 from crag in Perkin's Beach 170 yds west 

 of Shepherd's Rock (Grid Ref. SO 372999). 



Discussion. The most remarkable feature of the two biconvex, costate orthids 

 recovered from the Ordovician rocks of Shelve is that, although they belong to two 

 different genera from widely separate horizons, they are indistinguishable in most of 

 the attributes that are used to discriminate between species (Tables 30-38). Such 

 dimensional estimates are numerical expressions of the way certain exoskeleton 

 proportions were maintained during growth. They indicate the remarkable stability 

 of the primitive strophic shell commonly referred to as 'typically orthid'. But 

 there is no doubt that the two stocks can immediately be separated by the disposition 

 of the ventral vascula media which is the chief diagnostic difference between Ortham- 

 bonites and Lenorthis (Williams in Williams et al. 1965 : H311-313). 



Lenorthis proava was first described by Salter (1866 : 336) as a variety of Orthis 

 calligramma Dalman from the Arenigian Carmel Grits of Anglesey. The type 

 specimens have since been lost, but Bates (1968 : 147) has recently collected good 

 samples from the same horizon. Comparisons between the Mytton and Anglesey 

 shells show them to be the same in all morphological features except the costae 

 which are significantly coarser in L. proava s.s. This difference alone is not con- 

 sidered sufficiently important to warrant taxonomic recognition. It is, moreover, 

 noteworthy that other penecontemporaneous forms from Europe and eastern North 

 America may also prove to be morphologically comparable with L. proava. The 

 accounts given by Rubel (1961 : 173) of ' Orthambonites' orbicularis (Pander) and of 

 'Orthis' panderiana (Hall & Clarke) by Ulrich & Cooper (1938 : 102) suggest that 

 these stocks are Lenorthis closely related to the British species and especially similar 

 in the number of costae which appears to be the most reliable character for the differ- 

 entiation of species. L. parvicrassicostatus (Cooper) from the Upper Ordovician 

 of Scotland (Williams 1962 : 98) is also close to L. proava in the shape of its shell and 

 the number of its costae, but the external surface is not capillate. 



ORTHAMBONITES Pander 1830 



Orthambonites exopunctata sp. nov. 



(PI. 8, figs. 10-17 '> see a l so Text-fig. 3, p. 24) 



Diagnosis. Subcircular Orthambonites with the brachial valve 74% as long as 

 wide and 18% as deep as long and a pedicle valve 31% deep as long, ornamented by 

 about 15 costae pierced by radial rows of exopuncta, and with a wavelength of o-8 mm, 

 5 mm antero-medially of the dorsal umbo ; cordate ventral muscle scar extending 

 anteriorly for 36% of the length of the pedicle valve, dorsal adductor scars impressed 

 for 51% of the length of the brachial valve. 



Description. Ventribiconvex, subcircular Orthambonites with the pedicle valve 

 31% as deep as long and evenly convex in transverse and longitudinal profiles ; 

 brachial valve 74% as long as wide and 18% as deep as long with a shallow median 

 sulcus almost half as wide as the length of the valve but tending to die out anteriorly ; 

 ornamentation consisting of 14 to 16 subangular costae on 2, 4 and 3 brachial valves 



