PLATE 2 



Pigs. Nicolella cf. actoniae (Sowerby) 



1, 3. — Dorsal and ventral views of a well preserved indi- 

 vidual. Cleft chilidium, or chilidial plates, visible in 

 fig. 1. Compare angular costs and predominantly 

 concentric finer ornamentation with those of Orthis 

 (figs. 9, 11, 12, 15). 

 Ordovician (Lyckholm, Fj), Piersal, Estonia. Cat. 

 No. S151. x2. 



Nicolella actoniae (Sowerby) 

 4, 6. — Dorsal and ventral views of a large specimen, show- 

 ing anterior bifurcation of costs. 

 5. — Ventral internal mold, showing muscle impressions. 



Ordovician (Chasmops Is.), Ostragothia, Sodra Fre- 

 berga, Sweden. Cat. No. S 1 52. x 2. 



Nicolella, n. sp. 



2. — Dorsal view of a specimen related to A^. moneta 

 (Eichw.), showing coarse angular costs. 

 Ordovician (Wierland group, Echinosphsrites Is.), 

 Popovka near Leningrad, Russia. Cat. No. S 147. 

 x2.65. 



Orthis cf. calligramma Dalman 



7, 9, 1 1, 1 3. — Posterior, dorsal, ventral, and lateral views of 

 the same specimen. Note biconvexity of shells, 

 rounded costs, and parvicostells in the stris. 

 Middle Ordovician, Pulkova, near Leningrad, Russia. 

 Cat. No. 8 167. x 2. 



Orthis rotunda Pander 



10. — Dorsal interior, showing simple brachiophores, septum- 

 like cardinal process, small adductor impressions, and 

 pallial sinuses radiating from them. The sockets for 

 the teeth are the space between the brachiophores and 

 the wall of the valve. 



Ordovician ( Walchow) , opposite Iswos, Russia. Mus, 



Comp. Zool., Harvard Coll. x 2. 



Figs. Orthis rotunda Pander — Cont. 



16. — Dorsal interior, showing median ridge and pallial 

 markings. 



18. — Ventral interior, showing two prominent pallial trunks, 

 closely apposed and separated by a small, low ridge. 

 These trunks bound, on the inside, subreniform 

 ovarian markings. It is this type of interior that char- 

 acterizes the ventral valve of the Orthids as here 

 conceived. 

 Ordovician (Walchow) , opposite Iswos, Russia. Mus. 

 Comp. Zool., Harvard Coll. x 2. 



Orthis callactis Dalman 



8, 12, 15. — Posterior, ventral, and dorsal views of the same 

 individual. Coarse, simple, rounded ribs covered by 

 fine radii and finer concentric growth markings clearly 

 visible. O. callactis is the type of the genus Orthis, 

 and the kind of shell to which this generic name 

 should be restricted. Compare the outline and con- 

 tour with those of O. calligramma (figs. 7, 9, 11, 13). 

 A critical study of large collections of European Orthis 

 may prove the advisability of separating the O. calli- 

 gramma type as a subgenus. 

 Ordovician (Walchow, Glauconite Is.), Iswos, Wal- 

 chow River, Russia. Cat. No. S 144. x 2. 

 1 7. — Dorsal interior, showing small, simple brachiophores. 

 Ordovician, opposite Iswos, Russia. Shaler Mem. 

 Exped. Colls., Mus. Comp. Zool., Harvard Coll. x2. 



Cyrtonotella aflf. C. frechi (Wysogorsky) 



14. — A kind of biconvex Orthis with prominent concentric 

 lines covering the costs. 

 Ordovician (Echinosphsrites Is., C), Popovka, near 

 Leningrad, Russia. Cat. No. S 2099. x 2. 



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