GENERA OF THE SUBORDER ORTHOIDEA 



81 



Diductor scars divergent, elongate, not extending for- 

 ward of the anterior end of the delthyrial cavity; 

 adductor scars elongate, extended forward to the an- 

 terior end of the callosity. 



Dorsal interior. — Notothyrial cavity small, moder- 

 ately deep; cardinalia confined; hrachiophores not 

 distinctly separable from the supports; brachiophore 

 supports convergent at their junction with the roof of 

 the valve. A low median ridge extends forward 

 from the anterior ends of the brachiophore supports 

 nearly to the middle of the valve. Cardinal process 

 absent. 



Geologic range. — Ozarkian of North America 

 and Europe. 



American Species 



DalmartelU elecira (BiHmgs) 1862 

 D. eiecira Itrvh (Matthew) 1893 

 D. electra major (Matthew) 1893 

 Orlhis euryone Billings ? 1862 

 O. hipfolyte Billings 1862 

 Eoorthis fulillus Walcott 1924 



European Species 



Eoorthis Christiana! (Kjerulf) 1865 

 E.Jaunus (Walcott) 1905 

 E.tullbergi (Walcott) 1905 

 £. a»TO<j»«' (Walcott) 1905 



? E. bavarica (Barrande) 1868 



Distinguishing characters. — The distinguish- 

 ing characteristics of Archworthis are the dalmanelloid 

 exterior, large ventral muscle area, convergent brach- 

 iophore supports, and total absence of a cardinal process. 



Discussion. — Through the courtesy of the Cana- 

 dian Geological Survey it has been possible to study 

 Billings' specimens of O. electra, fortunately mostly 

 interiors, but all silicified. Therefore the microstruc- 

 ture of the shell could not be determined. O. electra 

 has been referred usually to Dalmanella but the inter- 

 nal anatomy does not justify its union with that genus. 

 ArchcBorthis most strongly resembles Paurorthis but 

 there are again certain anatomical diflFerences. In the 

 ventral valve the dental plates are much more clearly 

 defined in adult Archceorthis than in Paurorthis. Fur- 

 thermore, the latter rarely possesses the callosity in the 

 front of the muscle area, more usually it has a low 

 septal ridge or faint thickening that extends nearly to 

 the front margin of the shell. 



In the dorsal valve the chief internal differences lie 

 in the structure of the cardinalia and the absence of a 

 cardinal process. The cardinalia are never so ponder- 

 ous as in Paurorthis, and the hrachiophores are not so 

 long. In Paurorthis the hrachiophores are supported 

 by a swelling of the thickened notothyrial platform 

 around their dorsal edges, but in Archtrorthis there are 

 thick plates of adventitious shell that converge to the 



floor of the valve. The median ridge of Paurorthis 

 is much more sharply defined than that of Archa-orthis. 

 Externally the most notable differences are the lack of 

 marked fasciculation in Arch^orthis and its less in- 

 curved beaks. 



Archirorthis is confined apparently to the Lower 

 Ordovician or Ozarkian of America. In this respect it 

 differs also from Paurorthis, which appears in the 

 Glauconite sandstone, probably late Lower Ordovician 

 in age, but ranges into the Middle Ordovician (Echin- 

 osphasrites limestone). 



Although we are not yet able to point out a satis- 

 factory family relationship for Archa-orthis, it seems 

 best for the present to range it with the Orthidae near 

 Paurorthis. 



Subfamily PRODUCTORTHIN.-E Schuchert and 

 Cooper 1931 



A small group of aberrant and specialized Orthidae 

 characterized by exceedingly short interareas or none 

 at all. Both members of the family evolved a pro- 

 ductoid form which is carried to perfection in Pro- 

 ductorthis. The latter is a remarkable example of 

 homoeomorphy, foreshadowing a form of shell (but 

 without long tubular spines) that became dominant 

 in the Pennsylvanian and Permian. 



Panderina has been placed in association with Pro- 

 ductorthis because of the great reduction of its inter- 

 areas; it is clearly developing toward Productorthis 

 but internally has not yet reached the latter's stage of 

 high specialization. 



Accordingly, this subfamily has the genera Pan- 

 derina Schuchert and Cooper and Productorthis 

 Kozlowski. 



Genus PANDERINA Schuchert and Cooper 

 1931 



PI. 3, figs. 1-4 



Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 193 I, 

 p. 243. 



Genoholotype. — Productus abscissus Pander 

 1830, Beitr. Geogn. Russ. Reiches, p. 86, pi. 27, 

 figs. 7a-c. 



Description. Exterior. — Subsemielliptical, lateral 

 margins straight or concave ; hinge-line straight, car- 

 dinal extremities acute. Lateral profile piano- to con- 

 cavo-convex; anterior commissure broadly sulcate. 

 Ventral interarea extremely short, orthocline, ventral 

 umbo convex or swollen, beak curved, usually resorbed 

 for the passage of the pedicle ; dorsal interarea nearly 

 equalling the ventral interarea in length, anacline, 

 notothyrium partially closed by the cardinal process. 

 Ornamentation multicostellate, imbricate in front. 

 There is a tendency in this group for the ventral valve 



