80 SHELVE DISTRICT 



derived from Platystrophia by the superimposition of a multicostellate ornament. 

 The beginnings of a similar development can be seen in one external mould of 

 Salacorthis (PI. 13, fig. 12) but it was not necessarily typical of the species because 

 no costellae were developed in a larger pedicle valve and even in the figured specimen 

 costellae did not appear until the valve was about 7 mm long. The new genus 

 therefore differs from both Platystrophia and Mcewanella in being sporadically 

 costellate. 



The principal difference, however, and one which immediately distinguishes 

 Salacorthis from the other two members of the Platystrophinae, lies in the con- 

 spicuously exopunctate appearance of the shell surface. The exopuncta were deep 

 coarse pits penetrating the external shell vertically for depths of about o-i mm and 

 arranged in a closely crowded quincuncial pattern. They are quite different in 

 morphology and origin from the obliquely disposed apertures which constitute the 

 'hollow ribs' so typical of the plectorthids (Williams in Williams et al. 1965 : H70), 

 and are all the more unexpected because the external surface of Platystrophia is 

 noteworthy among orthaceans for being distinctively pustulose. 



Internal characters that indicate the platystrophiinid affinities of Salacorthis 

 include the simple cardinal process and the suboval ventral field which, although 

 undifferentiated in known specimens of the new genus, probably consisted of a broad 

 adductor scar flanked by elongate bases for the diductors and adjustors, as in 

 Platystrophia. The brachiophores and their supporting bases are also comparable, 

 although the degree to which the posterior parts of the dorsal adductor impressions 

 are inserted behind the brachiophore bases of Salacorthis is unmatched in Platy- 

 strophia and Mcewanella. But such internal differences as exist are minor and do 

 not affect the conclusion that Salacorthis arose out of the earlier established Platy- 

 strophia. 



Salacorthis costellata gen. et sp. nov. 

 (PI. 13, figs. 6-13 ; Text-fig. 7) 



Diagnosis. Dorsibiconvex, subquadrate Salacorthis with a brachial valve about 

 three-quarters as long as wide and 37% as deep as long ; dorsal fold almost half as 

 wide as valve length, bearing 2 or 3 costae with a wavelength of 1-5 mm, 5 mm 

 antero-medially of the umbo, with 2 or 3 on the flanks, all becoming costellate in 

 late growth stages ; ventral muscle scar suboval extending forward for nearly one- 

 third the length of the pedicle valve ; dorsal median septum short. 



Description. Dorsibiconvex, subquadrate Salacorthis with cardinal angles 

 becoming obtuse in adult growth stages, pedicle valve about one-quarter as deep 

 as long, brachial valve three-quarters as long as wide with a mean depth relative to 

 length of 37% for 3 brachial valves ; dorsal fold 49% as wide as the length of 3 

 brachial valves bearing 2 or 3 strong costae with a wavelength of about 1-5 mm, 

 5 mm antero-medially of the dorsal umbo, with 2 or 3 additional costae on each of 

 the lateral slopes, all costae may split into costellae in late growth stages ; exopuncta 

 coarse with counts of 3 per 0-5 mm, 5 mm anterior of the umbo. 



