ORDOVICIAN BRACHIOPODA 



69 



Fig. 6. Diagrammatic views of (A) the ventral and (B) the dorsal interiors of 



Astraborthis. 



submedial lobes of diductor scars impressed on the floor of a strong pseudospondy- 

 lium ; mantle canal pattern saccate. 



Dorsal interior with a simple plate-like cardinal process and short, blade-like 

 outward-curving brachiophores embedded in a thick deposit of secondary shell ; 

 sockets oblique, notothyrial platform weak, fused with median ridge ; adductor 

 scars quadripartite with smaller posterior pair ; mantle canal pattern probably 

 digitate. 



Type species. Astraborthis uniplicata sp. nov. from the Mytton Flags. 



Discussion. Although the new genus is obviously alimbellid in its general 

 morphology, it differs from other members of the family in its ornament and certain 

 aspects of its internal morphology. Externally it is immediately distinguishable 

 from the smooth Alimbella and finely capillate Medesia in its coarse costellation and 

 internally in the weak development of the notothyrial platform and the blade-like 

 nature of the brachiophores. It further differs from Alimbella in its wide ventral 

 adductor scar and from Medesia in the absence of branching and medial fusion of 

 the proximal parts of the vascula media. 



It may be of familial significance that dental plates have not yet been seen in 

 adult pedicle valves of any of the three genera assigned to the Alimbellidae. It is, 

 however, possible that short receding dental plates were developed during early 

 growth stages but were later entirely buried in thick deposits of secondary shell 

 secreted in the delthyrial cavity during formation of the pseudospondylium. Until 

 young shells or their impressions have been examined it is safer to delay assessing 

 the importance of this aspect of alimbellid morphology. 



