ORDOVICIAN BRACHIOPODA 79 



3 per mm, 5 mm antero-medially of the umbo and further corrugated by up to 7 

 radiating rounded costae with a wavelength of about 3 mm, 10 mm antero-medially 

 of the umbo. 



The ventral mould is distorted umbonally but there is evidence of an apsacline 

 interarea and massive trigonal teeth supported by strong, narrowly divergent dental 

 plates. The ventral muscle scar was elongately oval with a broad median adductor 

 track reaching to the anterior margin. 



The specimens are closely related to the Upper Llandeilo M. berwynensis (Mac- 

 Gregor 1961 : 183). However, the Welsh Mcewanella bears a ventral fold, and 9 

 superimposed costae, and until the variability of these features can be ascertained 

 in the Shropshire form it seems safer to withhold specific identification. 



SALACORTHIS gen. nov. 



Name. An orthacean with an external ornament like a sieve (Gk. adXai). 



Diagnosis. Subquadrate, biconvex, uniplicate shells with a strong dorsal median 

 fold emerging from a neanic sulcus and a complementary ventral sulcus ; radial 

 ornamentation coarsely costate to costellate with two or three thick rounded costae 

 on the fold and up to three equally wide costae on each lateral slope, costellae arising 

 sporadically in later growth stages either by division of costae or as intercalations in 

 the interspaces ; external shell surface also ornamented by densely distributed, 

 deep exopuncta arranged qnincnncially ; ventral interarea curved, apsacline with 

 open triangular delthyrium, dorsal interarea shorter, slightly curved anacline, noto- 

 thyrium open ; shell impunctate. 



Ventral interior with strong teeth supported by short but well-defined dental 

 plates extended on the valve floor as raised ridges laterally bounding an elongately 

 oval undifferentiated muscle field ; ventral mantle canal system unknown. 



Dorsal interior with a wide notothyrial platform bearing a median ridge-like 

 cardinal process, brachiophores rod-like, divergent, continuous laterally with strong 

 concave fulcral plates defining oblique, hemiconical sockets ; brachiophore bases 

 large, convergent onto notothyrial platform and median ridge ; dorsal adductor 

 scars divided by median ridge into a pair of triangular undifferentiated impressions 

 with the apices directed posteriorly and deeply inserted beneath the anterior borders 

 of the brachiophore bases ; dorsal mantle canal system unknown. 



Type species. Salacorthis costellata sp. nov. from the Spy Wood Grit. 



Discussion. The new genus, with the exception of a poorly preserved exfoliated 

 pedicle valve revealing the impunctate nature of the shell, is represented solely by a 

 few moulds from the Spy Wood Grit. Yet its affinities are not in doubt because it 

 combines the typical cardinalia of the Plectorthidae with the coarsely costate 

 ornamentation and strongly uniplicate anterior margin of the Platystrophinae. 

 Indeed with regard to its radial ornamentation it anticipates the relationship between 

 Platystrophia and Mcewanella (Schuchert & Cooper 1932 : 69). The former is costate 

 and the latter is believed, at least in respect of the American species, to have been 



