84 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



The great length of the cardinal process in Pro- 

 ductorthis is a necessary mechanical adjustment to the 

 great arching of the ventral valve for the manipulation 

 of the shallow dorsal valve. Elongation of the cardinal 

 process is a frequent phenomenon in shells having very 

 strongly arched ventral valves and flattened dorsal 

 ones, as in Triflesia, Chonetes, Productus, Plaiyorthis, 

 etc. In connection with the cardinal process, it is 

 worth while to direct attention here to the presence of 

 the small circular plate which covers the postero-dorsal 

 face of the cardinal process, but its function is unknown 

 to us unless it be regarded as the chilidium. 



Kozlowski, in seeking the origin of Productorthts, 

 looks, with correctness, to some form of biareagerous 

 Orthisy and selects as possible ancestors Productus 

 transversus Pander and its variety Orthis transversa 

 latestriata Lamansky from horizon Bi^. Both of 

 these forms are provided with areas but they are greatly 

 reduced. They likewise have simple, direct ribs such 

 as the progenitor of Productorthts must have had. So 

 far as the external development of Productorthts from 

 Orthis is concerned, these forms suggest the passage to 

 O. incurvata Lamansky, but since the internal features 

 of these forms are unknown it would seem best to retain 

 for the present the genetic descent noted by Kozlow- 

 ski. It is clear that Productorthts came from some 

 type of Orthis provided with interareas, such as Pan- 

 derinoy but this ancestor appears to be unknown. 



Subfamily ANGUSTICARDINIIN^ Schuchert and 

 Cooper 1931 



Subrostrate costate Orthidae apparently evolving 

 toward the rhynchonellids but still retaining interareas 

 on both valves and with more of the orthid characters 

 than of those of the Rhynchonellacea. Delthyrium 

 and notothyrium open. Cardinal process linear. 



Geologic range. — Early Middle Ordovician of 

 western Russia. 



Includes only the one genus, Angusticardinia Schu- 

 chert and Cooper. 



Genus ANGUSTICARDINIA Schuchert and 

 Cooper 1931 



(Lat. angustusy narrow, and car do y hinge) 



PI. 13, figs. 20-23; t. fig. 17 



Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1 93 1, 

 p. 244. 



Genoholotype. — Porambonites recta Pander 

 1830, Beitr. Geogn. Russ. Reiches, p. 97, pi. 11, 

 figs. 7a-e. 



Description. Exterior. — Shells rhynchonelloid, 

 with a faint fold and sulcus, margins rounded, hinge- 

 line narrow, cardinal extremities occasionally auricu- 

 late. Lateral profile biconvex; lateral commissure 

 straight; anterior commissure faintly uniplicate. Ven- 

 tral interarea longer than the dorsal but very short and 



narrow, faintly apsacline, umbo convex, beak curved, 

 delthyrium open. Dorsal interarea anacline; noto- 

 thyrium open. Ornamentation paucicostate but with 

 interpolation of costas along the margins. Test 

 impunctate. 



Ventral interior. — Delthyrial cavity deep; teeth 

 small; dental plates strongly developed, extended for- 

 ward as a thickening around the anterior margin of 

 the muscular area; umbonal cavities deep. 



Dorsal interior. — Notothyrial cavity moderately 

 deep; cardinalia confined to the umbonal region; 

 brachiophores short and bluntly pointed, supported by 

 strong plates which converge to meet a median ridge 

 that extends for about one-third the length of the 

 shell; cardinal process a linear ridge not reaching the 

 beak. Adductor field small, quadripartite. 



U7 O 



Fig. 17. — Sections showing the dental plates of Angusti- 

 cardinia. These ventral interiors strongly resemble those of 

 Rhynchotrema •plenum from the Chazy, but differ in hav- 

 ing a pronounced interarea. The sections have been cut 

 slightly obliquely. The section on the left is not far from 

 the beak, x 3. 



Geologic range. — Early Middle Ordovician of 

 western Russia, with the species Porambonites recta 

 and P. striatOy both of Pander. 



Distinguishing characters. — The distinguish- 

 ing characters of Angusticardinia are the very narrow 

 hinge-line, which produces a rhynchonellid outline and 

 profile in the shell, the small ears, paucicostate exterior, 

 and the ensemble suggestive of small Rhynchotrema 

 increbescens. On the inside the dental plates are 

 strong, and the structure of the cardinalia is unlike that 

 of any other orthid. 



Discussion. — These shells externally resemble the 

 rhynchonellids but differ from the members of that 

 superfamily in the possession of an interarea on each 

 valve, an open delthyrium and notothyrium, and a 

 cardinal process. This is the earliest rhynchonelliform 

 shell known and might, with more complete reduction 

 of the interareas and further narrowing of the hinge- 

 line, have given rise to rhynchonellids. All of the 

 fundamental structures are present for such an 

 evolution. 



Angusticardinia reminds one somewhat of Cyclocae- 

 lia among American shells but is older and had an 

 independent origin, probably out of some Russian 

 orthid. Although we are unable to prove this, we still 

 think it best for the present to leave these shells in 

 the family Orthidas as an independent subfamily. 

 Their evolution may have been into the lamellose 

 Ordovician rhynchonellids embraced in the genus 

 Rhynchotremtty as R. increbescensy R. cafaxy and R. 

 ferlamellosum. 



