ORDOVICIAN BRACHIOPODA 107 



Description. Small, semi-elliptical, piano- to concavo-convex Heterorthis with 

 cardinal angles approximating to 90 ° ; pedicle valve about one-tenth as deep as 

 long, slightly carinate medially ; brachial valve averaging 80% as long as wide for 

 5 valves, plane to gently concave with a slight median sulcus ; ventral and dorsal 

 interareas planar, apsacline and anacline respectively, delthyrium open, notothyrium 

 covered by chilidium ; radial ornamentation costellate with low rounded ribs 

 numbering 5 per mm, 2 mm antero-medially of umbo. 



Teeth small, supported by divergent dental plates ; ventral muscle field parallel- 

 sided and elongately bilobed extending forward for an average of 42% the length of 

 3 pedicle valves and only slightly narrower than long. 



Cardinal process consisting of a simple median plate with slightly splayed posterior 

 surface, embedded in well-developed notothyrial platform, brachiophores plate-like 

 acutely divergent, sockets simple without bounding fulcral plates ; adductor scars 

 elongately oval impressed on either side of low median ridge, extending anteriorly 

 for over half the valve length. 



Figured material. 



length width (mm) 



External and internal moulds of pedicle valve (BB 35423a, b) 4*0 - 



External and internal moulds of brachial valve (BB 35424a, b) 4-5 5-5 



Horizon and locality. Spy Wood Grit exposed 1100 yds NNE of Rorrington 

 (Grid Ref. SJ 303015)- 



Discussion. The Heterorthis collected from the Shelve area consist of only 12 

 valves from the Spy Wood Grit and one from the Aldress Shales. The largest of 

 these specimens was not more than 5 mm long and, although one would expect 

 some mature shells even in the small sample available, there is a strong possibility 

 that the collections are moulds of young shells. Indeed the undifferentiated cardinal 

 process and the relatively modest development of the notothyrial platform suggest 

 that this is so ; and until more is known about the age structure of such samples, 

 infrageneric recognition is withheld. In that respect the only clue to specific affilia- 

 tion is given by the relatively poor development of the lateral diductor lobes in the 

 manner of adult H. retrorsistria M'Coy (Williams 1963 : 420) although even that 

 character may be merely diagnostic of immaturity. 



TISSINTIA Havlicek 1970 

 1970 Tissintia Havlidek : 14. 



When Havlicek proposed Tissintia as a new genus, he rightly emphasized its close 

 relationship with Heterorthis. In consideration of the stratigraphic distribution of 

 the only species recognized by him as belonging to the genus, he concluded that the 

 stock bore many attributes of the prototypic heterorthid (Havlicek 1970 : 9-10). 

 The planoconvex to ventribiconvex profile, the bilobed cardinal process and the 

 disposition of the brachiophores in the even older T. prototypa confirm his identifica- 

 tion of the changes such features underwent to give rise to the characteristics of the 



