NO. 5 RHYNCHONELLOID BRACHIOPODS — COOPER 43 



Thomsonica Cossmann, Rev. Crit. Pal., vol. 24, No. 3, p. 137, 1920; Finlay, 

 Trans. New Zealand Inst., vol. 57, p. 532, 1927. 



Outline elongate-oval to triangular with the greatest width at the 

 front ; inequivalve, the brachial valve having the greater depth ; an- 

 terior commissure broadly uniplicate, the brachial fold low and in- 

 conspicuous. Surface marked by concentric lines of growth only. 

 Beak small, erect ; foramen minute, submesothyrid ; deltidial plates 

 conjunct, forming a concave plate. 



Pedicle valve interior with thick teeth attached directly to the shell 

 wall ; dental plates absent ; muscle field short and narrow, the di- 

 ductors small but surrounding the adductor scars. Vascula media 

 strong, branching about two-thirds the shell length from the beak. 



Brachial valve interior with deep corrugated sockets bounded by 

 long vertical socket ridges to which the long crura are cemented; 

 crura of falcif er type, crescentic in section, convex outward ; inner 

 hinge plates thick and filling the intercrural space. Cardinal process 

 small and transversely triangular. Median ridge short and low, but 

 thick, united with the cardinalia. Adductor field large, with large 

 anterior scars. 



Type species (by original designation). — Waldheimia (?) sinuata 

 Hutton, Catalogue of the Tertiary Mollusca and Echinodermata of 

 New Zealand in the collection of the Colonial Museum, p. 36, 1873 = 

 tTerebratula gualteri Morris, Quart. Journ. Geol. Soc, London, 

 vol. 6, p. 329, pi. 28, figs. 2, 3, 1850. 



Comparisons. — This interesting genus has exterior and interior 

 features that set it aside from nearly all other rhynchonelloids. It 

 is unlike all other known Tertiary and modern rhynchonelloids ex- 

 cept Patagorhynchia in the extremely small pedicle foramen and con- 

 cave deltidial plates. It differs from Patagorhynchia in being smooth 

 rather than marked by squamose costellae. Internally it differs from 

 all other known Tertiary and Recent rhynchonelloids except Frieleia 

 in the great development of the inner hinge plates which grow and 

 swell between the crural bases to plug the entire posterior. 



Geological horizon. — Upper Cretaceous to Miocene. 



Distribution. — New Zealand. 



Assigned species: 



Terebratula gualteri Morris. 

 Waldheimia ? sinuata Hutton. 



Discussion. — This genus presents some peculiarities not seen in 

 most of the Tertiary and Recent rhynchonelloids. The small foramen 

 is submesothyrid, an unusual position for this group of animals. The 



