38 SHELVE DISTRICT 



strong anterior development of the dorsal median septum and the strength and 

 anterior configuration of the ventral muscle platform. In these aspects, the new 

 species is certainly different from the type species, the mid-Caradocian Pseudolingula 

 quadrata (Eichwald) from the U.S.S.R. (Goryanski 1969 : 41), as well as various 

 Caradocian American forms (Sinclair 1945 : 59-62 ; Cooper 1956 : 215-217), and 

 other differences may become apparent in due course. 



Family ELKANIIDAE Walcott & Schuchert 1908 



MONOBOLINA Salter 1865 



Monobolina plumbea (Salter) 



(PI. 5, figs. 2-10 ; PI. 6, fig. 1) 



1859 Lingula plumbea Salter in Murchison : 50, foss. 8, fig. 1. 



1865 Monobolina plumbea (Salter) Salter in Murchison : 334, pi. xiB, fig. 10. 



Diagnosis. Subelliptical Monobolina ornamented by fila and costellae numbering 

 about 5 and 8 per mm respectively, 5 mm antero-medially of the umbo, and with well- 

 developed muscle fields extending anteriorly for about half the valve length. 



Description. Subelliptical, biconvex Monobolina with an evenly convex brachial 

 valve averaging 78% as long as wide and 13% as deep as long and a slightly less 

 convex pedicle valve tending to become flattened anteriorly in adult shells ; shell 

 surface except for protegular areas ornamented by thin sharp fila and costellae in 

 densities, 5 mm antero-medially of the umbones, of 3, 4, 5 and 6 fila per mm in 1, 2, 

 7 and 1 brachial valves, and of 7, 8 and 9 costellae per mm in 4, 7 and 2 brachial 

 valves ; ventral pseudointerarea roughly lamellose, orthocline, averaging 72% of 

 the width of the valve and 11% as long as the valve medially where it is divided by a 

 strong pedicle groove 52% as wide as long, dorsal pseudointerarea poorly developed ; 

 dorsal interior with a pair of variably impressed outside lateral and transmedial 

 muscle scars situated submedially 44% of the valve length anterior of the dorsal 

 umbo to form the postero-lateral boundaries to a pair of central muscle scars im- 

 pressed medially at about the middle of the valve, vascula lateralia poorly defined ; 

 ventral muscle platform low and solid with a convex boundary culminating in a sub- 

 dued median ridge extending anteriorly for 46% of the valve length ; three pairs of 

 scars representing the transmedian, middle lateral and anterior lateral muscles 

 occurring on either side of the median ridge and flanked by a pair of anterior lateral 

 impressions, vascula lateralia commonly well defined. 



Figured material. 



Internal mould of pedicle valve (BB 35467) 



External mould of pedicle valve (BB 35468) 



External and internal moulds of brachial valve (BB 35469a, b) 



External and internal moulds of brachial valve (BB 35470a, b) 



Internal mould of pedicle valve (BB 35471) 



Internal mould of pedicle valve (BB 35472) 



External and internal moulds of pedicle valve (BB 35473a, b) 



length 



width (mm) 



15-5 



20-4 



5-5 



7-5 



II'O 



- 



7-3 



10-0 



12-0 



15-8 



I7-0 



18-3 



12-5 



- 



