82 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



completely to enclose the dorsal valve anteriorly as in 

 Productorthis. Test fibrous, impunctate. 



Ventral interior. — Delthyrial cavity shallow; teeth 

 small; fossettes moderately deep; dental plates thick 

 and short, continued as a ridge around the front mar- 

 gin of the muscle area, forming a pseudospondylium. 

 Muscle field oval, small, occupying about one-third the 

 length of the valve and less than one-fourth the width. 

 In front of the muscle area is a low median ridge. 



Dorsal interior. — Cardinalia confined; crural bases 

 short; cardinal process a thick ridge projecting beyond 

 the level of the interarea; median ridge low, extend- 

 ing to the anterior margin of the muscle field. Muscle 

 area small, anterior adductors the larger, and lobate as 

 in Productorthis. Margin crenulated, anterior margin 

 produced into a ridge as in Productorthis so that 

 the dorsal valve fits into the ventral valve as an 

 operculum.*^ 



Geologic range. — Lower Ordovician (Bie-Biio) 

 of Europe. 



Species 



Productus abscissus Pander 1830 



P. abscissus var. 



P. extensa Yandtr 1830 



P. latus Lamansky 1905 



P. tetragonus Pander 1830 



Discussion. — The genus Panderina is intermediate 

 between Orthis s. s. and Productorthis. Orthis char- 

 acters are seen in the short, curved ventral interarea 

 and incurved beak, and the narrow, plane interarea 

 of the dorsal valve. Internally the simple cardinal 

 process shows the genetic connection of the genus. 

 Aberrant characters from Orthis toward Productorthis 

 are multicostellate exterior and imbrication of the ribs 

 in the anterior of the shell. The strong ventricosity 

 of the ventral valve and the shallowness of the dorsal 

 valve have produced a crenulated flange on the inside 

 of the upper valve which serves to hold the shell 

 firmly in its articulation. 



Internally the resemblances to Productorthis are a 

 little more remote. The ventral muscle field suggests 

 the latter genus in its elevation on a deposit of adventi- 

 tious shell and in the short dental plates. An orthoid 

 cardinal process is retained. According to Lamansky's 

 figure there is a suggestion of lobation of the anterior 

 adductors which is a typical Productorthis feature. 



Panderina appears earliest in the Walchow forma- 

 tions of Lower Ordovician age (Bi) and is one of 

 the oldest of the orthid genera. Its range extends 

 from Bi to Biia, where it disappears and Productorthis 

 appears for the first time. It is not improbable, then, 

 that Productorthis arose from Panderina. 



*^ Description of dorsal interior drawn from figures in 

 Lamansky, Mem. Com. Geo!., n. s., livr. 20, 1905, pi. 2, 

 fig. 12. 



Genus PRODUCTORTHIS Kozlowski 1927 



(An orthid having the form of Productus) 



PI. 3, figs. 9, 11-16; pi. 4, figs. 15, 16 



Kozlowski, Bibl. Univ. Lib. Polons, fasc. 17, 1927, p. 9, 

 pi., figs. 7-9c. 



Genoholotype. — Productus obtusus Pander 1830, 

 Beitr. Geogn. Russ. Reiches, p. 87, pi. 26, figs. 7a-c. 



Description. Exterior. — Productoid, semicircular 

 to subquadrate; hinge-line long and straight, fre- 

 quently mucronate; cardinal extremities auriculate; 

 lateral profile concavo-convex to plano-convex to un- 

 equally biconvex, the ventral valve always with far 

 greater convexity; anterior commissure rectimarginate 

 or very broadly sulcate. Cardinal interareas absent or 

 remnantal; ventral umbo swollen; beak curved, not 

 uncommonly resorbed for the passage of the pedicle; 

 dorsal beak obsolete; ornamentation multicostellate, 

 rugose, the rugae being extended as frills. Test fibrous, 

 impunctate. 



Ventral interior. — Delthyrial cavity deep, teeth 

 strong; crural fossettes deep; dental plates strong; 

 muscle area borne on a thick platform, longer than 

 wide ; adductor and diductor scars not separable ; ad- 

 justor impressions (?) situated at the base of the dental 

 plates, small and elongate; pedicle callist a small scar 

 in the apex. Posterior margin grooved to form a 

 ginglymoid joint into which the posterior margin of the 

 dorsal valve is inserted; anterior margin crenulate. 



Dorsal interior. — Cardinalia confined to the pos- 

 terior margin ; brachiophores short, of the Orthis type, 

 supported by a deposit of shell substance under their 

 anterior extremity. This deposit encloses the anterior 

 of the socket so that the crural bases appear to be spoon- 

 shaped. Sockets long and deep. Cardinal process 

 elongate, extending behind the posterior margin. It 

 has a stout, long shaft and thin carinate myophore on 

 its postero-dorsal face. The myophore may be covered 

 by a small circular plate, probably a modified chilidium. 

 A prominent median ridge extends forward nearly to 

 the anterior margin of the lobate muscle area; an- 

 terior adductors the larger, each scar being divisible 

 into a smaller and a larger unit; posterior adductors 

 very small, subrectangular, separated from the anterior 

 pair by a low ridge at right angles to the median ridge. 

 Anterior margin thickened to form a dorsally crenu- 

 late flange in order to fit into the ventral valve. 



Geologic range. — Lower Middle Ordovician of 

 Europe and South America. 



European species. — The following species of 

 Pander's Productus (1830) belong here, together with 

 two other species described by later authors: 



Productus aculeatus 

 P. hrevis 

 P. costatus 



