PLATE 3 



p,Q5 Panderina abscissa (Pander) 



1 , 4. — Dorsal and ventral views of a specimen belonging to 



the genotype, showing characteristic ornamentation, 



especially concentration of growth toward the front. 



Note reduced (shortened) ventral interarea well shown 



in fig. 1. 

 Ordovician (Glauconite ss.), Popovka, near Lenin- 

 grad, Russia. Cat. No. S 157. x2.25. 



Panderina tetragonum (Pander) 



2, 3. — Dorsal and ventral interiors. After Lamansky, Mem. 



Com. Geol., 1905, pi. 2, figs. 11,12. 

 Ordovician (Walchow, Bi^), Popovka, near Lenin- 

 grad, Russia. Slightly enlarged. 



Paurorthis parva (Pander) 



5. — Ventral interior, showing teeth, crural fossettes, median 

 ridge, and lateral ovarian areas. The muscle area 

 shows a broad adductor-diductor impression. Cf. 

 Dalmanella rogata, pi. 17, fig. +. 



7. — Dorsal interior, showing subradial pallial sinuses, indis- 

 tinct muscle field, and prominent elevated median 

 ridge. Brachiophores oblique, divergent plates sup- 

 ported by a swelling of adventitious tissue, to be seen 

 at posterior portion of median ridge. Cardinal process 

 a faintly discernible ridge just behind the swelling of 

 adventitious substance. Cf. Ddmanella rogata, pi. 17, 



fig- 31. 

 8, 10. — Ventral and dorsal exteriors of a large mdividual. 



Note reduced (shortened) ventral interarea and sub- 

 fasciculate costellx. The general external resemblance 

 of P. farva to Dalmanella is a striking instance of 

 homoeomorphy. Paurorthis, however, differs intern- 

 ally as illustrated above (figs. 5,7). It differs also in 

 being impunctate (no endopunctas) but possesses nu- 

 merous exopunctae which have led to its misidentifica- 

 tion as Dalmanella. 

 Ordovician (Glauconite Is.), Gornaja Scheldicha, 

 Lake Ladoga, Russia. Cat. No. S 1 36. x 3. 



6. — Ventral interior, showing broad pallial trunks sepa- 

 rated by a low median ridge. Subreniform ovarian 

 areas occupy lateral portions of valve. Wide adductor- 

 diductor track clearly visible and adjuster (?) im- 

 pressions outside these may be seen. The interior of 



Figs. Paurorthis parva (Pander) — Cant. 



Paurorthis is thus close to that of Orthis s. s., and the 

 genus forms a remarkable homoeomorph of Dalman- 

 ella s. s. 

 Ordovician, opposite Iswos, Russia. Shaler Mem. 

 Exped. Colls., Mus. Comp. Zool., Harvard Coll. x 3. 



Productorthis parallela (Pander) 



9. — Ventral interior, showing reduced (shortened) inter- 

 area, prominent dental plates, and peculiar muscle 

 field. The small, elliptical impressions at the base of 

 the dental plates have usually been interpreted as 

 diductor scars; they are, however, smaller than is 

 common for the scars of the diductors and probably 

 represent adjustor muscle impressions. The diductor- 

 adductor impressions occupy the large central elevated 

 area. 

 13, 16. — Ventral and dorsal exteriors, showing imbricating 

 lamellx or "ruffles," remarkable "productoid" form, 

 and nearly obsolete interareas. 

 Ordovician (Chazy, Glauconite Is.), Gornaja Schel- 

 dicha, Lake Ladoga, Russia. Cat. No. S 126. x 3. 



1 1 . — Dorsal interior, showing elongated cardinal process 

 with its compressed myophore. The brachiophores 

 have developed into cuplike structures which receive 

 large teeth. Circular chilidium visible on dorsal sur- 

 face of free or posterior end of cardinal process. 

 Bipartite character of anterior adductor impressions 

 clearly visible. The ribbed elevated border around 

 the periphery of the shell is an aid in articulation. 



12. — Reverse, or dorsal, surface of fig. 11, showing imbri- 

 cated exterior and small, umbrella-like chilidium cov- 

 ering free end of cardinal process. 

 Ordovician (Chazy, Glauconite Is., BJ, Popovka, 

 near Leningrad, Russia. Cat. No. S 127. x 3. 



Productorthis cf. eminens (Pander) 

 14, 15. — Ventral and dorsal exteriors of a species having 

 finer ribbing. The "ruffled" exterior, however, is 

 well exhibited. In fig. 15, ventral interarea nearly 

 obsolete, and a rounded foramen formed by resorp- 

 tion of the beak by the pedicle. 

 Ordovician (Glauconite Is., BJ, Popovka, near Len- 

 ingrad, Russia. Cat. No. S 122. x 3. 



200 



