108 SHELVE DISTRICT 



Caradocian heterorthids. The reflexed costellae along the posterior edges of the 

 shells of all heterorthids are especially interesting because well-preserved specimens 

 from the Weston Beds show corresponding follicular embayments indenting the 

 hinge-lines of both valves. These embayments indicate the presence, at some time 

 or another, of backward-projecting setae. Whether functional setae persisted 

 much within the cardinal angles is doubtful because the embayments may have 

 been incorporated within laterally expanding hinge-lines after they had ceased to 

 accommodate setae. 



With regard to the origin of the heterorthids, the existence of so typical a Tissintia 

 species as prototypa in the Lower Llanvirn suggests that Paurorthis, the earliest forms 

 of which are not much older, may not be ancestral to the Heterorthidae as Havlicek 

 (1970 : 16) believed. Indeed as in many other brachiopod groups, proposed lines 

 of descent can be no more than tentative guesses until more is known of the 

 Tremadoc-Arenig faunas. 



Tissintia prototypa (Williams) emended A. W. 

 (PL 17, figs. 15-19 ; PL 18, figs. 1-9, 11) 

 1949 Dalmanella prototypa Williams : 168. 



Diagnosis. Subcircular, ventribiconvex Tissintia with fine costellae reflexed 

 posteriorly along the hinge-line and numbering 5 per mm, 5 mm antero-medially of 

 the dorsal umbo ; brachiophore plates parallel with median ridge with bases extend- 

 ing anteriorly for 15% of the length of the brachial valve and 110% of their lateral 

 spread ; ventral muscle field elongately bilobed, extending forward for 38% of the 

 length of the pedicle valve. 



Description. Subcircular, ventribiconvex to rarely planoconvex Tissintia 

 with obtuse cardinal angles ; pedicle valve 21% as deep as long with an evenly 

 convex median zone and flattened lateral areas, brachial valve 80% as long as wide 

 and about one-tenth as deep as long or rarely planar, shallow median sulcus dying 

 out on average 5 mm anterior of dorsal umbo (variance 0-016 for 10 valves), lateral 

 areas flattened or slightly concave ; ventral interarea planar, apsacline, longer than 

 anacline dorsal interarea, delthyrium and notothyrium open ; radial ornamentation 

 fascicostellate commonly 5 per mm, 5 mm antero-medially of the dorsal umbo, 

 external branches poorly developed in the first four sectors, costellae reflexed postero- 

 laterally so that corresponding follicular embayments indent the internal surfaces of 

 the hinge-lines of both valves. 



Teeth small, trigonal, supported by dental plates extending anteriorly for 14% of 

 the length of the pedicle valve and diverging for twice their length ; ventral muscle 

 field elongately bilobed divided by a pair of fine median ridges extending forward 

 from conspicuous pedicle callist ; adductor scars small, elongately oval, submedially 

 situated ; diductor scars impressed anteriorly for 38% of the length of the pedicle 

 valve but not enclosing adductors, adjustor scars impressed on the median surfaces 

 of the dental plates ; mantle canal pattern lemniscate. 



