ORDOVICIAN BRACHIOPODA 43 



representatives of Apsotreta in the Shelve succession. The valve which had a 

 subcircular commissure just under 3 mm in diameter was conical in profile and about 

 two-fifths as deep as long with a planar apsacline pseudointerarea and an apical 

 foramen ; the mould of the pseudointerarea was not sufficiently well preserved 

 medially to determine whether an intertrough existed. The ornamentation con- 

 sisted exclusively of fine growth lines. Internally the parallel-sided apical process, 

 so diagnostic of the genus, extended forward for about two-fifths of the length of 

 the anterior slope between 2 pairs of simply disposed vascula lateralia ; a pair of 

 cardinal scars were impressed on the posterior surface. 



The valve cannot be profitably compared with those of other described Apsotreta, 

 but it is noteworthy that its occurrence greatly extends the stratigraphic range of the 

 genus which had previously been regarded as restricted to the Upper Cambrian 

 (Rowell in Williams et al. : H276). 



CONOTRETA Walcott 1889 



Conotreta stapeleyensis sp. nov. 



(PI. 6, figs. 16-21) 



Diagnosis. Conotreta with a gently convex brachial valve averaging 85% as long 

 as wide and a conical pedicle valve 37% as deep as long with a procline pseudo- 

 interarea bearing a low deltoid arch ; apical process transversely oval, submedial 

 vascula lateralia not deeply divided. 



Description. Medium-sized Conotreta with a very gently convex brachial valve 

 averaging 85% as long as wide and 6% as deep as long (for 3 valves), and a conical 

 pedicle valve averaging 37% as deep as long (for 3 valves) ; pseudointerarea well 

 defined averaging 71% of the maximum width of 3 pedicle valves and procline or 

 rarely nearly catacline in attitude, so that the beak with an apical foramen may be 

 located forward of the posterior margin by as much as 20% of the valve length, 

 pseudointerarea divided medially by low deltoid arch with a mean width of 30% 

 of its length ; dorsal beak small, situated at the posterior margin defined by an 

 anacline pseudointerarea ; external ornamentation consisting solely of fine growth 

 lines. 



Ventral interior with a pair of subcircular cardinal scars impressed postero- 

 laterally of a transversely oval apical process, mantle canal system pinnate with the 

 submedial branches of the vascula lateralia not deeply divided. 



Dorsal interior with conspicuous propareas defined by a depressed median platform 

 extending forward of the umbo for 18% of the length of the brachial valve and 

 continuous with a long thin median septum of unknown profile ; cardinal and 

 anterior muscle scars forming a quadripartite pattern about the median septum and 

 divergent vascula lateralia and extending anteriorly of the umbo for 56% of the valve 

 length. 



Type material. 



length width (mm) 

 Holotype Internal mould of pedicle valve (BB 35566) 2-5 3-0 



